I’m not a full time nerd anymore, so don’t understand the ins and outs - but my take on developing DLSS and FSR is that these are technologies created by the relevant GPU manufacturers for their respective hardware.
From a game development perspective you want to cater to both sides of the fence, but shoe horning an open source rendering practice in to your engine can’t be the easiest task in the world. So I can absolutely imagine these GPU manufacturers lobbying them, “we’ll fund x amount of your development time, and assign developers to help you implement this. However you can’t arrange a comparable deal with any of our competitors”. So I would imagine the lesser implementation of FSR would be down to the dev team having to work it by themselves.
Pure speculation, but an entirely reasonable guess in to how these practices run.
This is far more likely the reason rather than "intentionally" gimping one side. That "appears" to be the difference between nvidia and amd when it generally comes to these things, nvidia seem to want active input and have a real say in how, where and when to apply and use said effects where as amds approach always comes across as "here's our solution, open source, use as you please" i.e. over the fence, not our problem anymore. Of course, nothing wrong with either approach and there are pros/cons to both methods.