Why though? Is there something stopping developers from using the plugin/easy way of implementing dlss??? If the hard part has been done for them then why would they go and try to do it from scratch when they can just do the easy way???
Not disputing that FSR is easy from the get go but to completely blank out that dlss is also just as easy, if not easier to enable/add in unreal engine, unity based games is silly when that is not the case as per developers comments.
And again, don't disagree that FSR will eventually take the lead in the amount of games it is in but as it is right now, dlss is in more games and in games which need all the perf. they can get.
Its Marketing speak, it is easy to implement but getting the quality out of it is a lot more involved. You're asking a valid question and the answer is not something along the lines of "yeah its just a case of installing it and then pushing buttons, but we cannot be bothered"