[cpitman]

I think what may be missing that this is more than just a "graphics" library, and that by taking over the event loop and overall structure of the app they make it so that you can get a running game (as opposed to rendering a quad) far faster.

Game development forums are full of skeletons of people that tried to roll their own "game engine", forgetting that their goal was to make a game[1]. Unity (haven't used Unreal yet) makes it so that you can get a prototype done in less than a day. I probably looked like a kid waving my phone around like a joystick to steer my sub around a 3D ocean, changing the handling, etc. Starting with a graphics library would make that a multi-day task.

[1]: No really, when I was younger I essentially stopped working on my game and developed a fully compositing GUI/Window manager. Fun, but totally besides the point.