Or there's this integrated seatpost/saddle design Tom Ritchey made 40 years ago.

https://www.youtube.com/watch?v=BwdDA-0iNhc

Don't know how you would adjust that though. I guess it could be made to slide fore-aft, but angle? Buy a fixed angle for the wrong seat tube and you're out of luck.

But anyways, my point is, it would probably not be too ridiculously difficult to engineer an integrated seat/post, even one that is adjustable, but the saddle does not come off of the post.