Because it’s not actually reducing any overhead. What you get it fewer high-fidelity “really” frames each second, in exchange for roughly 2x (or more) low-fidelity “fake” frames

So a 60 FPS game before may run at 100 FPS after, which is really only 50 real FPS + 50 fake FPS.

Also some of the frame generation algorithms are tied to upscaling, so textures and everything are loaded in lower res, and an algorithm guesses what’s missing.

The more you let the computer guess what’s supposed to be there the faster it runs but the less accurate it gets.