Because Make-Up Air (MUA) is meant to replenish the area with fresh air (O2 mainly), and in some cases to pressurize the area. If you have make-up air (whether it is to be an air curtain or not) near a kitchen hood exhaust, this fresh air "short-circuits" the area and you are still left with stale air for the most part (and difficulty pressurizing if that is a secondary purpose of your MUA)..