Why do the Packers only rush three men all the time? That strategy killed them when they lost to Pittsburgh.?

Why do the Packers continue to do this? They're the only team in the NFL that goes with the wimpy three man rush no matter what the situation. You would think that in some of those situations, most teams would have at least four guys rushing the QB. But the Packers have this strategy called "make the opposing QB as comfy as possible and don't bother the man because he is trying to compete a p." Roethlisberger had all night to toss the football in the last drive. In fact, he had all night all night long to complete his pes. The guy could've even had enough time to eat few hot dogs and put mustard on them before throwing his p. If he is going to be comfortable and well rested in the pocket he may as be well fed too. Why does Dom Capers insist on sticking with the three man rush no matter what the situation is? Sometimes a situation does call for a blitz.