Sure, you can sleep in a plush seat that unfolds into a flat bed in First Class. But can you afford it? me neither. You may also sleep in a wide and comfy chair in business class, but is it worth the steep price premium? Probably not (unless your boss pays for it). So here you are, sleeping in a coach seat. 
But why? why did the airline industry standardize on minimally reclining seats with limited legroom? Airlines would love to cram as many seats as possible into an plane to maximize their profit. Alas, the more seats you fit in, the less comfortable they must be. Although they may like to, airlines can’t go all the way and install narrow, back-less benches for passengers to sit on or simply leave them standing up.
The more comfortable the seat is, the more money air carriers can charge for it. This is evident by the exorbitant amount often charged for business class tickets. The cost of the slightly better in-flight service can hardly justify it. The bulk of the cost can only be justified by the increased comfort level of the seat. To maximize their revenue, airlines continuously optimize the choice of chairs and the distance between them. They have zeroed in on a certain sweet spot in terms of seat comfort and size. 
This equilibrium has been maintained for decades, with all air carriers using similar dimensions.
So next time you find yourself dosing off in a chair that clearly wasn’t designed for sleeping, remember that there’s a good reason for that. By twisting and turning in discomfort throughout a red-eye flight you’re helping the airline industry stay afloat.