You have to play with 2 CBs if you want Pirlo to be unmarked. When you play 3 CBs, that decreases the numbers we have in attack so the opponent is comfortable to sacrifice a man for Pirlo. If you take out a CB and add a man in attack, we could potentially punish them much easier for sacrificing a man to Pirlo since our numbers in attack would increase.
blah blah blah, anyways, Conte knows this I'm sure. I think he's comfortable with holding possession at the halfway line for 90 minutes and squeaking out a single goal.