My cat constantly steals food from my toddler (and anyone else who isn't looking!) why?

My cat is almost 5...he has been a food thief for as long as I can remember. I do not allow him on my counters, and do not ever voluntarily give him people food. He gets fed 2x a day ...and not measely portions! He isn't overweight, he isn't underweight. His constant thievery is going to get him sent to a new home here shortly, I'm done with it. If I walk out of the room for a second when my toddler is eating, he's allover her food like white on rice. He'll steal a morsel & then go run & hide under something to eat it. Lord knows how many times he got away with it before she could talk! Now she hollers whenever he does it. If he even thinks no one is looking, he'll try it. I've been sitting in the same room, reading a book or occupied with TV & he'll do it. When I see him, all hell breaks loose ...I chase him down to snatch the food away and give him a good swat. I'm over it. The cat gets fresh food & water daily. I've even tried different brands of food, he likes them all. Not picky, not sick, just a healthy, annoying cat. Why is he like this & why can I not stop this behavior???