It's also a lack of specialized talent. There's lots of great "talent" at game devs and even middleware devs. There's just not much great talent that deals with renderers and API development. The vast majority of devs just lean on the middleware developer to push out the renderer codebase. In a situation like Bethesda running their own studio engine, they just don't have the right people for it. This plagued the 90's when people were trying to code for Glide, OGL, DX5,6,7,8, and 9. Many studios folded because they couldn't get their tech to work with hardware acceleration.