Airlines, trains, taxis, ferries and all other modes of transport charge on a per seat cost, even though all modes of transport experience marginally greater cost as the total weight of passengers and loading increases.

Which is all fine - the bit I object to is when they take my seat as well as their own.....

they could make the price on all of them based by weight but most of them are cheaper then airplane travelling, none of them consume as much fuel as an airplane for same load

There is also the fact that, price-per-seat, has become the global standard in all modes of transport (I expect there will be one or two exceptions) but if one company decided to try a different pricing method - they would have to change things at the airport, as discussed above. They would have to make these changes at every airport they served but, if no other carrier changed to this method?

We know that the airlines spend a great deal of time thinking of clever ways to charge us more - if this was viable - they would have done it.

Changing airports is not impossible. The move from queueing up at a desk for checkin, to some airlines now charging a fee for not entering passport details on the web has meant major remodelling of many airports

200 hundreds and some years ago the British developed the imperial measuring system wich seems to from another era, but now they are adopting also the metric system , manny old people are still talking about their weight in stones, wich is funny because stones can be in manny sizes.
The new generation are learning the metric system also, so we should always look for improvement, if we want to change something we can do it.

You're not related to Priti Patel, by any chance ?

:O

Humans change things when enough humans want them to change. If there are not enough in favour, the change fails (perhaps a revolution fails too). Sometimes thing change as they are manifestly simple and obvious.

Moving to a unified method of counting and measuring is in the second category. Trade benefits. Everyone benefits.

For better or worse, the world has agreed to travel being based on a 'seat from A to B'. How that is priced varies enormously but, since 1945 (or so) - weight of pax has not been part of the price calculation. On any occaision that this topic is raised in the USA - more pax complain than support it. If you can find a way to change the USA way of pricing airline seats to the benefit of the airlines then you are sitting on a gold mine. If it only benefits the pax - forget it.

Weighing passengers can be done in manny ways, they could put scales at checkpoints, they could make built in scales in seats, we only need more people to be aware of this, and there can be a change