It's hard to find a way to sit that will keep you comfortable for a long period of time. This is especially true on long distance busses, but is not in any way limited to those. Indeed it is a problem we've had to deal with since leaving the water. For even though that was arguably a breakthrough in evolutionary engineering, which certainly must have employed many of the very best genes at the time, it has left us removed from that supporting embrace so that we have to fight through every living moment not to collapse from gravitational stress.
The most logical way to approach this problem is that taken by the snakes. Spread the weight over a large surface and minimalize the pressure put on any single part. A good idea, so naturally evolution has tried it out and kept it.
But still we weren't satisfied, we wanted up in the world. So somehow legs were invented. Not the brightest idea really, but it once again gave evolution something creative to do with all those millions of years. Figure out how absorb the stress and the shocks inherent in not only concentrating the weight on a few points, but in lifting and moving these points one or several at a time.
And now to the final test, can it be done with only two contact points? It would require some serious computing to constantly monitor and adjust such a creature so it wont overbalance and fall, possibly falling prey to one of it's many legged cousins. And would two legs really be sufficient for the task of keeping any useful amount of mass away from the lovng embrace of Mother Earth? In the spirit of natural selection not much thought (in fact none) was given to such considerations and a few of models were quickly slapped together over the course of a few million years (tens or hundreds, something like that). These were apparently successful for a good long while, multipied and ate many of their four footed friends (it was already in style among creatures large enough to make a decent meal not to have more than four legs). But in the end they proved unequal to the task and died out. It must be said in their defence that many creatures with a greater amount of legs (even a giant centipede or two) threw in the towel and walked out of history at the same time.
Call it bullheadedness or stupidity if you like, but evolution will keep trying, in new ways and old. New models were soon n the making. One given wings that don't work and made too stupid to care and the other given an opposable thumb and a big energy consuming brain as week compensations for the pain and suffering.
So here we are today, still trying to use that brain and those hands to make comfortable bus seats. So that we at least for a while, say for example the while it takes between Sendai's and Shinjuku's respective stations, can again approach the problem of gravity along the way of the snake.
Beds and large tanks of water have been found to be too big and/or heavy to be economically viable alternatives to seats on a bus.

To conclude: Japanese bus seats aren't all that bad, a bit narrow (the guy who had the seat next to mine before he went and found another would surely agree) but there's plenty of leg room. Now I just wish I'd had the sense to bring my tea and snacks in with me instead of leaving them in my bag in the luggage compartment.

Listening to: Blue Öyster Cult - E.T.I. (Extra Terrestrial Intelligence)
Feeling: Stiff and bored