The US is a place of capitalism, and that imho includes the toilets.
There is only what is strictly necessary so to not waste money and of course nothing that would make people be there longer then as fast as possible.
After all, McD made seats that get uncomfortable after a certain time to "nudge" people to make place for new ones.
I would think that capitalism would also provide incentive for creating a better toilet experience. Better products often lead to increased demand, but the bottom line and customer turnover to play a big role in business decisions.
I'm only guessing here, but I think the full-length doors on the stalls in Japan, which I think is their best part, were developed to close off the hole-in-the-floor toilets. If those toilets had half-doors, everybody would see you doing your business. If I'm right, using the full-length doors on the newer toilets is just a practice that has been carried over out of habit and custom.