> What's curious is why there isn't a standard/de-facto standard (like JSON based one maybe?) for UI layouts that has emerged

It hasn't emerged because nobody has done it. Nobody has done it because it's unbelievably hard and involves a lot of painful code of the sort that is not fun and therefore that people must be paid to write. Nobody has been paid to write it because nobody pays for dev tools and it's not in the best interest of any platform vendor to commoditize platforms.