[Ekaros]

I think it's reasonable to remember that Half-Life 2 or Source goes back to days when Valve was still just a gamestudio. So they didn't have the resources they have now. Yes original Half-Life, expansions and Counter-Strike were popular, but Steam for one wasn't a thing. But they weren't printing money yet. So spending time on engine with a ambitious game wasn't something they had money or time for.

And then Global Offensive is title originally developed by third-party. So I don't think those guys either were going to put huge effort in engine. And once it got going, there is little push to replace it again. Specially when it's raking massive amounts of money in even in the state it is...