My weight has bounced up and down a bit over the past few years, and while I've never been obese (just overweight), I very much notice how gaining weight begins to restrict my range of motion, so I think it is often a bit of a snowball effect. As you gain weight, moving gets harder, and when moving gets harder, you probably move less, making it even easier to keep putting on weight.
Like you mentioned, I've also noticed that my pain levels often increase when I move less. I was very hesitant to run or do squats for a long time due to a family history of knee problems, but I have a strong suspicion now that those knee problems were likely made worse due to inactivity. I am still pretty cautious about any knee aches, but I found my knees became less achy as I worked to strengthen my leg muscles.
I think car culture is also a major factor. I currently live in a location where I can comfortably function without a car. I do make an effort to exercise for the sole purpose of exercise, but even if I didn't, daily life still forces me to get up and move around quite a bit. Not too long ago, I was visiting family and borrowed the car to run some errands. I had 2 stops that were less than a block apart, so I decided to walk, and I will never make that mistake again, because the way everything is designed there really encourages driving and punishes pedestrians.