Each game usually has their own engine, while DLSS/FSR is done by Nvidia/AMD and everybody uses either Nvidia, AMD or Intel GPUs, so the overhead in developing time for the game devs is bigger in optimizing the game vs supporting DLSS/FSR.

However I agree with the sentiment, I wish game studios invest more effort actually optimizing their game instead instead of doing the lazy “let’s just add DLSS/FSR support and call it a day”.