Levels as well would take less time to develop, everything is dropped randomly in MP games unlike SP which has specific drop points for enemies and objects to interact with etc. (Even more so for Deep Down since the levels make themselves)
Other games using different engines humans still have to create the multiplayer levels and populate them with things.
Can people stop complaining about the lack of SP? It's getting old real fast. MP is where it's at for this one so time to move on.
SQUEEEEEEL!!!!! Looks so amazing! Good stuff Maynard