There are roughly around 700-800 PS4 Pro enhanced games out there. It took around 6 years to reach that point as well. Adding PSSR is easy true, but PS4 Pro literally just required something as simple as boosting your resolution.
It required updating the game to the latest SDKs for that though - PSSR can be added without it.
Also way too many PS4Pro enhanced titles just did the bare minimum - getting any of the Pro specific enhancements (like CBR) was actually rare.
It has some frame blending techniques to try to mask interpolation, but it is not creating new pixels.
That's not how CBR that Sony introduced works - at all.
But all that aside - spatial algorithms(interpolators, and AI ones) are 'creating' new pixels - temporal algorithms (AA and upscalers alike) don't - the whole point is to
reproject samples from previous frames, not
create what never existed in the first place like a spatial algorithm does.