Because a leak in the inlet manifold is introducing un-metered air into the engine, so air the MAF has not detected, so the ECU does not know about. When my breather was split, my engine hunted massively at idle when cold, almost to the point of stalling, but was pretty much OK when warm. The guy in Ford even said they're surprised how many problems a little pipe can cause!

Edit-Ignore what I said about a MAF if it hasn't got one as Marty said.