The way some emulator use the gpu can be quite different from the way pc games use it.

You can look at this article: https://dolphin-emu.org/blog/2017/07/30/ubershaders/ for an example how console graphics programming differ.

Middleware designed for games don't allow nearly enough flexibility to handle all the possible edge case that can arise. Console gpu (and graphics api) and computer gpu don't map one-to-one and they need all the feature and support from the underlying api that they can get for replicating the console way of doing things, supporting a new graphics backend is a lot of work for this reason. middleware try to abstract common pattern and flows, but every console is quite unique (in some way) middleware are not build for that and often are (a form of) a lowest common denominator for graphics api. Which is fine... for games.

edit: some clarification