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.