system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do animals at the zoo often look very healthy and in shape, despite a relatively sedentary life style?

I don't see the Tigers running anywhere, but they're still relatively healthy in weight. 

I also notice monkeys virtually never look fat, and more often than not, look plain muscular.

Yet the humans there are always super fat. What's the deal? It must be more than just diet.
gpt
The animals get exactly the right food, in exactly the right amount and they are very closely supervised and monitored by animal medical experts.

If the humans lived in a cage where the only food they had to eat was the food their doctor thought would make them the most healthy, they would not be fat.

If the humans had nothing to do all day except play with enrichment items provided by their doctor to enable them to work out and maintain muscular physiques, they would look muscular.