It's very easy to gain weight, especially with all the fast food options and temptation. Obesity is also often the consequences of many other problems affecting a person. Eating can easily become a coping mechanism, just like drinking, smoking, drug abuse, pretending the enjoy the nintendo switch, and so on. It's a form of self abuse.
It's also a problem that sneaks on you over the years. You start getting bigger. Exercising becomes harder and harder. You start to get older too. You start developing more and more problems (back problems, diabetes, etc). Everything gets harder. Life, in general, gets harder. Then you start developing self-esteem issues, it affects your work, then you start to experience sleep apnea, and so on and on, it's an avalanche of shit slowly drowning you.
And yes, some people are also just total morons and eat themselves to death because they're idiots and think that eating pizza 5 times a week is OK
I think it's a pretty multi-layered problem that just can't be easily be fixed by saying 'Stop drinking gravy lol'. It's like telling people suffering from depression that they should 'smile and stop being so bummed out".
Nutrition and exercise are tools to solve this problem, but a big part of the battle is treating mental health problems, personal issues and also identifying the causes of those problems. Just like substance abuse. If you don't want to patch the holes and just pump the water out, the boat will sink eventually.
If you can't fix that personal problems part, the physical aspect of the battle is like playing on nightmare mode.