They (Valve) had also to think about performance. You just can't have a big map the whole time in memory and let it render every frame, you have to stream the map. Having thousand of functions running every frame with their own stack is also a no go for performance.
The straightforward way of just pushing everything in the level and then blaming the engine if it fails might look easy and like a way worth going, but reality is that you have to think too. Sorry if this destroys your view of the world now.
First of all, I'm not professional such as Valve guys... Should that mean that there is no way for me to make cool and fast FPS game?
I've made really fast movement and weapons (did everything I could do to make it fast!), but placed grass models and 20-40 fps?!
