Why don’t many low cost airlines offer ZED fares or ID90s
 

The answer is simple: ZED and ID90 fares have to be reciprical between airlines!

I think you are actually right, it doesn't really matter as you only go on when a seat is available . And on my last company a ZED ticket was even more expensive than a normal ticket, if you booked that a bit in advance. So the company could probably even make some money on it.
But one reason might be, that most LCCs are not IATA members, which isn't a requirement for Staff travel, but it seems like most IATA members have ZED/ID90 non members don't.

I have made very, very little use of airline staff travel when I have had it available to me for the simple reason that it is so very unreliable. Airlines are doing everything that they can to fill seats with confirmed bookings so the chances of getting a 'spare seat' are very rare. Having spent 11 hrs on a cabin-crew jump-seat and many nervous hours wondering if I would get a seat, for a price that is the same as, if not more than, the cost of a confirmed seat booked 'on-line', that cheap confirmed seat is "worth" far, far more than any ID90 ticket.

The airline world has changed. If you plan carefully, cheap seats are available. ID90s are now just far too unreliable to have any real value.

In a certain way that is true. ID Travel is not anymore for going on holiday with your family. Confirmed tickets cost little more, but a lot less stress.
ID tickets are more used to commute with some options available and they simply give you the flexibility in case of roster changes.

You must have had some bad staff travel agreements or have been traveling on underserved routes that were always full... In 4 years with ID travel available to me I've saved thousands of euros for myself and even traveling with friends. Recently I flew halfway around the world for €70 on a full service carrier and back in business for €200 (normal price €3800 or thereabouts). You have to be a bit more flexible for sure, but to say it has no real value... Even low-cost long-haul carriers couldn't beat those prices. If you apply your careful planning to the ID tickets you can usually go a much longer way for the same or less money ;).

Are you retired? Just wondering how the flexibility fits in with time available.

For us the certainty of a confirmed seat is worth the price (and often cheaper if you end up with unplanned hotel costs added on top of your staff travel ticket cost).

We have travelled right around the world on confirmed tickets booked well in advance at a good price when we did have staff travel (with one of the major Alliances -- there's a clue!) available to us, Many asked us if we were going to use staff travel and we said that we were specifically ruling that out because we didn't want to risk wasting time and money with the uncertainty. We wanted that certainty to help us enjoy the trip.

Not retired, I just plan my vacation with a buffer day or maximum two on each end and usually plan travel via airports with scheduled traffic from multiple of the airlines that we have agreements with (not too hard to do, without detours mostly). But I have not yet encountered a completely full flight once. Actually, that's a lie. I had an overbooked flight once due to an earlier flight getting cancelled. That did mean a 5-hour wait, but that is what the flexibility is for. We do also have a few partners that allow "confirmed" ID tickets. A bit more expensive but a bit more certainty as well. Have only used it once though.

In terms of hotels, I either budget for whatever period I am planning to stay and/or book stays with free cancellation until shortly before arrival. If I end up arriving a day later, too bad but no real money "lost" in my view. I guess the value in ID90 tickets is there for this kind of approach. I can also see your point of view, believe me, I just thought it was kind of harsh to say it has no value.

Apparently it's more of a Europe thing. Most major low costs in Europe like Ryanair/EasyJet/Norwegian/Wizz Air don't have ID/ZED agreements with other airlines. The only exceptions I remember are Germanwings and Vueling, but because they're part of Lufthansa Group and IAG respectively.

But if you think on a worldwide perspective there are quite someLCC with ID/ZED available.

Canada: WestJet
USA: Southwest, JetBlue, Frontier
South America: GOL, SkyAirline
Asia/Oceania: Jetstar, Flydubai, Tigerair

It basically boils down to the fact it costs money to run a Zed arrangement and requires some administration expenses, time to co-ordinate deals etc. There's no incentive for them to do this when it's seen as an additional, unnecessary cost.

While that might work traveling as a couple or alone, it get's pretty nerve-wrecking with a family unless everyone is old and mature enough to be sent and wait alone and maybe even do part of the trip alone. It has admittedly become a lot easier with online ticket booking but even traveling alone I've ended up on the last jumpseat and prefer a bit more expensive booked staff tickets. A day here or there for flexibility is quite a lot if summer vacation is limited to two weeks. Airport fees are quite a big part of ticket costs here (and charged for SBY as well), which reduces the difference between SBY and confirmed.

There is the answer. A LCC have no interest to give it away for 70 but would have sold it to someone for 3800 or thereabouts.

I understand that in at least one European country discounted staff travel is taxable.

Well it would be quite hard to get a standby ticket e.g at Wizz Air where no such category exist. If there are no free seats left you can't book a ticket. If there are empty seats then your booking (be it positioning, christmas ticket, bonus ticket or any kind of company travel) is OK and confirmed. You learn to appreciate this when the average load factor reported is over 90%.

That's not true, there could easily be quite a large imbalance between loss of revenue...

For example. many would want a Virgin Atlantic/Easyjet travel swap. The Virgin guy could reasonbly commute on easyjet easyjet staff travel (upto 2 flights a week?) but for the easyjet guy, it would just be for a holiday maybe once a year on Virgin.

If the VA guy is using easyjet twice a week, that's potentially a massive loss in revenue at easyjet. Of course, staff travel swaps would probably account for most of those circumstances anyway but it would certainly be a net loss for easyjet.

I'm an easyjet captain and would of course love one of these schemes, our staff travel is pretty good tbf. You can often get confirmed tickets and if you book a standby, confirmed staff travel tickets are regularly released so it could possibly convert into a confirmed. Also, our confirmed staff travel tickets (EXACTLY the same price as a standby) are privy to EC261 compensation which is incredible tbh for a ticket that's £5+taxes.

In the US, a far larger number of pilots commute from their hometown to whatever major hub they've been shoved in so a better commuter scheme is more valuable there.

I realise that it's lovely to have reciprocal travel arrangements in place, but that kind of undermines the entire stratagy of low-cost, which is to disrupt/defeat the legacy carriers legacy.

That depends on country. In Germany it is not tax free in general, there is a personal tax break for discounted stuff (including all discounted things because of work), but if you go above that you have to pay taxes.

That said, i did enjoy staff travel with either direct reciprocal discount or ZED tickets all over the world, and especially so the chance to use business travel on longhaul flights as that makes longhaul flights bearable to begin with. Now working in an airline that has no deal with any other airline (we have internal staff travel though, both standby and confirmed bookings) i have to adjust there and search for normal tickets like anybody else in the general public, and that is no problem either.