You mean like Flutter? Flutter is similar to a game engine in that it draws every pixel itself originally using Skia but now also using it's own custom render e

You mean like Flutter? Flutter is similar to a game engine in that it draws every pixel itself originally using Skia but now also using it's own custom render engine called Impeller on some platforms. So it can work. However using something like Unity to make a UI app has a whole host of problems, many of which other commenters have already mentioned: power/CPU, poor accessibility, difficult to implement things, etc.