I think people don't use 3.5" floppy drives any more. Or parallel ports.
So yes: the UEFI boot spec is FAT. Once, we didn't have UEFI and maybe once, in the future, boot spec won't be locked in FAT.
Edit: @Guy claiming exFAT was developed for flash drives. Not sure if your clever spindoktering is astroturfing or trolling. >_<
All of those are good things in isolation. However, the reality is, Linux/BSD/etc. doesn't have the marketshare to push any filesystem cross platform.
Any filesystem with the characteristics you want that goes cross platform will start on MacOS (unlikely) or Windows (much, much more likely).
Until then, people need a way to accomplish tasks and exFat is the best we've got today for cross platform flash storage.
* I suppose it's possible it could come from Android or iOS, although iOS is pretty unlikely since it's so hostile to the idea of filesystems. For this to happen, though, it would have to be a regular task to plug flash drives into phones. That is currently not the case and won't be the case in the near future.