OK thinking about the actual question, why are we obsessed with food.
I've just remembered saying similar to this on another thread, but I think I might be seen as obsessive about food, because in our culture, eating shit is the norm. Anyone who deviates from that culture, is, therefore, slightly more noteworthy than the norm and is therefore seen as obsessive.
I'm actually not obsessive. I strongly believe that children and people in general should eat mostly real food, not processed food prepared in a factory with lots of additives, salt and sugar. Because of this, I guess a large number of people would see me as obsessive. But actually, I could ask them why they're obsessively buying biscuits, crisps, sweets, chocolates etc., every week in their shopping trolley. They would be very surprised to be described as obsessive for buying what they see as "normal" food. Whereas for me, it's not normal at all, it's stuff I'd buy for a children's party, not everyday use. And because of that, they'd call me obsessive...
So I guess what I'm saying, is that I disagree with the premise of the original OP. One mother's obsessive is another person's norm.