For a while I've been wondering if the push to DX12 or vulkan as the "better" APIs has been a factor in the big engines becoming a near monoculture with games development. Games are very varied in what they require, some push the limits but many releases are more modest yet lots of them are gravitating around full featured leading edge Unreal/Unity. Having a lower barrier to entry for graphics programming that lets them make something that's a closer fit to their requirements.
The other big push would be Epic cutting royalties until you're earning a significant amount, which would encourage studios not to hire or allocating as much resources to in-house.
I don't really think it is related. Graphics aren't really the most difficult part of a modern engine, and there are high quality open-source 3rd party solutions for rendering anyway.
In fact the "engine" part itself is quite small compared to the editor, and the hardest things can be done with third-party solutions, a lot open source: physics, rendering, audio, ECS, controls, asset loading, shader conversion.
The reason people gravitate towards Unity/Unreal is because of the low barrier to entry. This caused the monoculture among hobbyists.
The reason studios are gravitating to those engines is because of there is plenty of cheap labour available.
The other big push would be Epic cutting royalties until you're earning a significant amount, which would encourage studios not to hire or allocating as much resources to in-house.