Because it costs $1000/mo and insurance wants to make it as hard as possible to get that covered because they cannot afford to pay $1000/mo * 45% of Americans without doing things to their rates that are forbidden by the ACA and, for that matter, any approximation of good sense. If people cannot afford an additional $450/mo each on average for their health care, how do you cover a critical long-term $1000/mo drug that 45% of the population needs?

Gating it behind mandatory expensive, difficult-to-schedule appointments with a specialist who is in abruptly short supply where the insurance company is doing their damndest to kick as many of them off their network as they can without getting caught to keep the shortage going is certainly part of that strategy. And the result is “people do not stay on the drug”, which is their goal, and if they don’t meet that goal they have an even bigger problem and can’t continue to exist as a functioning company.

Because the pharmacy will refuse to sell it to you.

Source: UK based friend who says the pharmacy will refuse to sell them once they fall under BMI 25 (still overweight). They'd prefer to be on the tiny maintenance dose but it seems to be very hard to achieve (unless you're going off the market completely).

Cost is a big factor. When it becomes generic then I suspect people will stay on it for a lot longer.

You can also think about it the other way - if a drug caused you to lose 10-20% of your body weight, and you're now at a good body weight after 2 years, why would you want to lose another 10-20% body weight?

I understand that's not really how it works, but people often go very much by feel more than anything else.

Side effects? Also many might not be able to afford it long term as it's quite expensive.

Because they're now "normal", so why would they continue paying for it, taking unpleasant injections, and enduring the side effects?

In this sense it's like any diet: they "work", but if you don't permanently modify your food intake, the weight comes back as soon as you go off the diet.