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.