Theres a bit of confusion here, the engine SUPPORTS breakables but it doesnt incorporate them (you can't just break everything, you have to choose what you can and cant break during creation of a map). Breakables are entity based and the maps are limited to the number allowed. I once tried to make the Lobby from The Matrix as a CS map, with lots and lots of breakable panels on the pillars, and after about 50 they stopped showing up and acted as invisible objects until I broke some of the existing ones.
The Half-Life engine does not support destructable/deformable
terrain so it's out of the question.
Lag is also a serious point, heck i've seen some computers struggle when a grenade breaks glass in Half-Life multiplayer, so a kamehameha breaking a rock or two would be hell. It would also mean mappers having to re-edit all the existing maps (in .RMF/.MAP not .BMP format) and making each rock breakable. Now consider the explosive nature of ESF. Lots of rocks breaking, respawning only to be broken again, by several people on a map, it's going to lag like a mofo.
In a nutshell it IS possible to have breakable rocks but would take up lots of time, effort, organisation, machine power and bandwidth to achieve when it's much easier just to hit your enemy and not the terrain.