You missed my point. These "game engines" are toolkits for artists that are built to be pretty and interactive.
I think that's the wrong approach. I would design an "engine" from the first line of code to be clean and high performance. I'd use external tools for assets, and I care more for incredible performance on all machines (i.e. 120+ fps on an old computer) rather than the industry target of 30 (sometimes 60) fps on a modern computer.
The behavior of the game should always come before making it shine, and I think you can only achieve that if you approach it from the right direction. These modern game engines aren't doing that (in my eyes)
But, as I said, I'm harshly critical and idealistic.