I'm not sure your second point works, or maybe I just don't understand it. It's not like the doctor is making judgements that people are fat outside a hospital- they're doing their job. You've got a car and it's starter goes out every year, last time being a year ago. Your car wont start. Whats the first assumption?

It's not ableist or bias to assume that the most common issue is the most likely issue. They see a ton of people whos problems are irrefutably due to their weight. It's not the doctors job to make judgement calls on whether that person is wholly responsible for their situation, it's their job to doagnose the problem and help take steps to fix it. The problem being their weight, the steps include: burn down capitalism and replace it with a system that doesnt incentivise companies to use the cheapest least healthy ingredients, or tell the patient unless they lose weight they're going to die. One of these is completely pointless to tell the patient, the other gives them an unfair opportunity to potentially save themselves.

which points squarely at an environmental cause

No, it points to people eating processed food and other shit. Guess what, you can still be healthy if you eat healthy.

Yeah but your doctor cant prescribe you burning down capitalism, they can prescribe you lower your caloric intake.

The environmental causes are availability of options we crave but are still not forced into, so individual responsibility is absolutely a thing.

I was obese and it sucked but I got down to a healthy weight, and keeping it off kind of still sucks but it doesn't take a lot of time or money, in fact it's generally cheaper.

Fast food is constantly highlighted as an impossibly unhealthy reality, the nicer places cost more and take too much time. Except you can choose passable choices in fast food.

If you can freely pick, there are fast food places that offer salads with maybe some grilled chicken, which can be healthy unless you opt to drown it in ranch.

But let's say you are in a group and they pick a restaurant without an option like salad. Just asking for water instead of a big sugary drink gets you so much closer to healthy. Skip the fries, skip the mayo, get a smaller burger. All these things are cheaper and friendlier to a reasonable caloric budget.

It sucks because it means eating to feeling "ok" while skipping the most awesome foods and rarely getting to feel just utterly full, but that was just life when people had healthier weight.

Similarly on activity. It does suck that work has people sedentary, but our idle pursuits are similar. When I was a kid, TV was stuck on a schedule and video games were only so engaging, so we would get bored and want to do something. Maybe it was walk amongst some trees to see if anytime interesting was around. Maybe do something with a ball. Nowadays we can get endless engagement from streaming, video games, and Internet. So tempting to just be on the couch. We can still choose those more active things, but we don't want to.

Note all this awesome stuff is still great in moderation. I just went full on gorging at a restaurant a week ago on pretty much whatever I wanted. The thing is this is maybe like once every 2 or 3 weeks, not daily like we really want to.