I've actually found a bit of a drop in comfort on airline seats at least in economy class, due to the use of leather/faux-leather rather than the cloth seats they used to use - that may also be due to fire regulations. Nonetheless I'd agree they're definitely more pleasant than most seats fitted to UK rolling stock and I imagine a lot of that is due to price. The point about resale value sounds very plausible too, but also because airlines are in competition with each other. If you use the train somewhere in the UK, in the majority of cases, you're stuck with one particular operator. Because of this, in the few rare cases TOCs are in competition, they compete on fare prices rather than experience. The airline industry is far more fiercely competitive and the onboard experience, at least to most carriers, is hugely important.
There's also the matter that nothing in the aviation industry is cheap, and there simply aren't the same sort of companies operating in the aviation sector as make bus and train seats.