They’re not forced it’s more like they are habituated to it. Just like every smoker knows cigarettes are bad for them, every fat person knows a soda and fries are bad for them. But it’s a temporary escape from the rest of their problems so they succumb to it more easily than people who are better at delaying gratification.

Many places justify a “sugar tax” on certain fast food items like large sodas, to disincentive those purchases. that combined with cheap available healthy options may remedy the obesity epidemic somewhat. It’s tough to justify what’s healthy/unhealthy food, though, especially because portion size is hugely important.