It's not that there isn't a benefit to under eating. Demonstrably there is.
Nor is there a strict disadvantage to overeating sometimes - it keeps you alive and is a great way to store food.
It's just that we probably regularly did both. We probably ate a lot, then very little, either on daily, monthly, or annual cycles. So it just so happened that beneficial things took advantage of those cycles as the right time to do beneficial things.
Think of it this way: I need to defrag my hard drive, but I cannot do that when I'm playing games. It's not that there's a benefit to stopping playing games, it's just that that's when I can defrag. It's probably similar with all the autophagy benefits of fasting - we were probably regularly not eating so we did it then. During eating we can't really do all that good stuff.
And conversely, I really need to build muscle and brain tissue, and that's probably easier to do when I'm calorie rich. It's just the right time, and I can count on a calorie-reduction later to do the other stuff.
Overeating doesn’t cause any issues to any animals during their average reproductive period (younger ages). Longer lifespans and calory deficits lead to lower quality offsprings. Nothing “demonstratable” there.
