They put a moral value on being a certain weight and they feel they "earned" it. They are angry that someone else might get that same social currency, as they see it, without doing what they did to "earn" it. Never mind that to get prescribed these medicines, you will have more than "earned" it, if misery and poor mental health surrounding food and body image is the yardstick, which of course it is. So if you are to get the social currency, they need to devalue it somehow, which they'll do by essentially claiming that you gained it immorally. You cheated. You don't really deserve it. It's tainted.
It's nothing to do with health, physical or mental; if it were, they'd be glad that there's an option to try when mere diet and exercise hasn't worked long term. They'd also understand that mental health matters too and it might be worth carrying some extra flesh if it prevents mental breakdown - fat people still deserve to be happy, ffs. It's about placing a moral value on something that doesn't have one and then trying to devalue it in others so they can still feel superior.