Because FSR3 is really new, released Sep 2023. Even the games that actually can patch in FSR3 didn't get to do it properly until Avatar nailed it and just earlier this month released another update to push it further. In short, AMD is working with devs to improve their plugin integration to various engine devs, and I don't think 3.1 is the "end goal".

FSR 3's result really depends on how developer understand and work with the proposed render pipeline compare to DLSS(which basically runs AI kernal to guess what pixel values to fill). Especially with games that features pip scope(fake UI scopes with on the fly fov changes are fine) or some translucent elements where it can not do the velocity buffer properly. (basically most of the fringeness on edge or flickering/swimming are mostly from precision, and ghosting are from wrong velocity when you see the old FSR artifacts).

You can replace DLSS 3 with FSR 3 with a mod but yeah, it's weird that so few games officially support it