Because people constantly walking around the cabin isn't going to hinder the FA's as well as annoy other passengers. Not to mention queues for the microwaves. Plus how many meals do you take? What if some greedy guts has 4 or 5? How would such a thing be policed so to speak.
Personally I've never had a problem asking FA's for a drink or snack if they don't happen to be doing a service at the time. Meals have come a long way recently and I can't think of one bad meal in recent years. That is 'good for a plane meal', which can't really be compared to restaurant cuisine so enough of the allegations that I clearly don't know good food.