Because what they are sold is a dream, a fantasy. Have a look at all the marketing from the big schools and its all images of pilots walking through the terminal in uniform. This is what a large number of wannabes want. They will say it's all about the flying and they just love to fly but if that was the case why are so many already in their 30's with 50k available for pilot training but not even a PPL? Any marketer knows you sell the dream not the reality.
It was always going to be the case that the job of pilot eventually lost the glamour and took on the same kind of image as a train or bus driver. We probably had another decade or so of people (and banks) willing to pay huge amounts to train for something that might pay them back in 10 years. What we will see now however will be a quick decline in T&C's and paying to train will just not be viable. Pilots on old contracts will be given an option to leave or sign a new one, the legacy carriers will offload a ton of experienced crew and the loco's will be rubbing their hands at how cheap they can pick them up.
The training industry will be decimated. No money or lending and parents in negative equity will be the end of that. We might see a couple of large schools remain as sponsored training comes back but it will be very, very compettive and require the pilot to remain with the sponsoring airline for a very long time on very low income to pay for it.
As for new airlines starting up, possibly but you wouldn't want to work for them. It's more likely that existing ones downgrade even further. BA will be more like Ryanair and those A380's will become cargo haulers. We are seeing a big push towards businesses embracing technologies that allow remote working. The mindset has been forced to change and many are now getting rid of the big office already as they realise the savings to be made. Why use their electric, water, rent when the worker can use their own? The business travel sector will never come back to what it was.
People will still fly though but it will mostly be the leisure travellers and they all want to pay less for their flights than they do for their airport parking.
There will however be plenty of other opportunities to make money if your smart enough. There will also be a large amount of capable light aircraft going very cheap. If you really do really want to fly then make your money outside of Aviation, get qualified and fly yourself around IFR on your own time. The only money that will be made in aviation over the next decade is by those shorting the market.