It's interesting that this phenomenon is much more prevalent in game development, as opposed to (for example) web development: some people build their own web development frameworks, but it seems like every game dev tries to make an engine at some point. My theory why: good web frameworks consider developer experience to be of paramount importance, and invest heavily into examples, documentation, and API improvements…
There's also the issue that most engines are built to be good at a specific game type and may make trade-offs in terms of how assets are managed and scene complexity. An open-world has much more different requirements than an on-rails shooter. For instance I had to do some pretty heavy retrofitting of one of those engines to handle LoD/geometry streaming and a bunch of other optimizations to make them work well with a large sight-distance game.