Lightning Round

Ok, this has probably been discussed a billion and ten times on here, but I've never quite understood something about the Lightning Round in the Who Knows Each Other Better game. Ross clearly states at the beginning that the rules are "30 seconds, all the questions you can answer." Both teams miss a question and Ross immediately moves on. In theory then, couldn't the girls have made a wild guess, like saying he's a firefighter, knowing it's wrong, just to save time and move on to another question? Why does Ross say "You need this or you lose the game?"