Because it's pretty likely some of these have negative long term effects. And some of those long-term effects are probably pretty serious.

Many people taking these kind of drugs are gambling with their well being. No doubt that in many cases that gamble is worth it.

But issues that can be solved with non-pharmaceutical solutions, such as with weight loss and lifestyle changes, are really better solved that way if possible. There's no risk of longterm effects and it helps foster a larger culture of healthy living in society generally, reducing the lilihood of the problem in the general population.