If I had to hazard a guess it would be because humans aren’t built to ride bikes. 
Also, keep in mind there are some things that just can’t be universal. Shoes come in a lot of sizes and widths. If you get the wrong size, it hurts.
Really though, it seems like in order to support better, there would be so much material interfacing with your body that mobility would be severely hampered. Redesigning the bike is an option - recumbents are very comfortable with their stock seat. EliptiGO bikes take your butt out of the equation entirely.
I guess I’d have to admit that I’m stumped on how to change the saddle without making it bad in other ways. Do you have any ideas?