Yeah but currently, CAS sucks and the Tank doesn't exist, and almost all the fighting STILL takes place in the damn caves. The caves will always give the xenos an advantage and for whatever reason every map(except maybe trijent, from the one time I played it(all of prison counts as caves, fuck prison)) includes massive cave networks. They're so big that the xenos have no real reason to come out of them until the marines have been totally smashed and are retreating. The caves are deep enough that you can coat the whole place in sticky resin and then just let the marines push in. They bleed themselves dry because fighting in that environment absolutely sucks for them, but they're so weak defensively that they need to push in anyway because they can't get the momentum back if they lose it. You turn the caves into a nightmare maze of thick walls and sticky resin, dark and full of blind corners, and with their CAS and arty unable to reach, and then you just wait for them to push in and get murdered. If the place is setup properly and you have boilers the marines have no hope of actually reaching the hive before they take enough losses to break, and the marine tools that are designed to help that situation, and are only useful when it comes to fixing that situation, don't work because of the cave.
If the cave networks were smaller, they would serve to protect the most vulnerable and valuable xenos, like the queen, drones and larvae as well as wounded xenos, from marine strategic weaponry(mortars, OBs, CAS) while letting it still be useful against combat oriented xenos, who would have to leave the caves to be useful. As it stands, the xenos have no reason to leave until they plan to take the Alamo, and at that point the marines have been torn apart and are just trying to leave.