I did some volunteering on an open source game.

Game goes through various phases of:

    A) This system is suboptimal but works
    B) Let's rewrite suboptimal system
    C) Oh god, we forgot our rewrite to propagate, so it's combination of A and B
    D) This system doesn't do anything let's delete it

And this is a very unexciting game. I.e. it uses 2D for everything. Imagine for a moment you now to deal with:

    E) nVidia and rayTracing
    F) unreleased consoles
    G) consoles that are old enough to write
    H) OpenGL, DirectX, Vulkan, Mantle, etc.
    I) Marketing trying to hype gamers by promising Pie in the Sky
    J) Multiple platforms