There are a lot of reasons, if we can separate them generally into two categories:

1. What the actual fat does to the body
2. How people become obese

For the first - The hormones released by fat can cause many problems, the fat itself can literally squeeze and crush the internal organs (that is what happens with OSA), the weight ruins your joints, etc..

The second - No one becomes morbidly obese by eating carrots and apples. You need to consistently be eating really shit food for a long time in high quantities to become truly obese (theoretically it is just calories in and out but realistically you can’t actually overeat on cabbage). These types of foods have a great impact on hormones, your blood vessels, your gut microbiome, and so many other factors that basically negatively impact almost every system in your body. This paired with being less mobile (also due to step one) can have a dramatic impact on your cardiovascular system.

To really explain everything would take literally hours, but basically these two categories are responsible.