Games typically need to redraw the entire screen anyways. Imagine a sidescroller where even a 1px movement requires the entire background and every object on top to be redrawn, or in 3D even a tiny camera move shifts the position of every triangle on the screen. Even very simple games without a lot of movement often have animations, where you then have to worry about transparency so you may have to redraw the region under the animated element anyways even of it doesn't move, etc. You could do a diff hoping for the happy occasion of a mostly-static frame, but a) that gives a performance penalty to the majority of frames that would need a total or near-total redraw anyways, and b) would lead to either unstable FPS or pointlessly hitting the FPS cap.

There are exceptions to this, for example a visual novel or a turn-based strategy game, but those are often satisfied to just eat the performance penalty for the ability to use standard tooling, or can use engines or standard UI toolkits like React that can do a temporal diff. For example, I've written several text-based games [1] that just use the DOM because using canvas or WebGL wasn't necessary. I've written other games that use a hybrid approach of absolutely-positioning temporally diffed DOM over a frame-by-frame redrawn canvas, to get the best of both worlds [2]. And I've also written games in e.g. Unity where it's easier and performant enough to use ImGui and redraw it every frame. Ultimately it cones down to what's practical and what's performant enough.

[1] https://vgel.itch.io/themengi , https://vgel.itch.io/the-sacred-text [2] https://vgel.me/hoverator