Some times the owner has mental health issues.

A pet doesn’t get fat on its own. It’s about separating the pet from the abusive owner.

If you have multiple pets that can be harder than it sounds. And if you don't have a ton of money the good solutions (e.g. locked feeders with RFID tags) can be pricy.

It doesn't completely work that way, just like for humans. Sometimes feeding pets less is just subjecting them to pretty severe discomfort and hunger, while their metabolism is deciding that food is scarce so they better hoard every calorie they can spare. I know it's significantly urgent to help them lose weight because of the health impacts, but IDK that it is super simple once you've decided to try to make it happen.