People like comfort too much.
You need to say "NO" to a lot of things if you want to be fit and healthy, you need discipline, you need dedication, you need to get out of your comfort zone.
Most people aren't willing to do that, sitting on your ass all day and eating tasty processed food is too comfortable, it makes you feel good, at least temporarily.
Forcing yourself to eat healthy food which might not taste as good or physically exerting yourself on a regular basis doesn't.
Most people only like the idea of being fit and healthy, not the long tedious process.
You can see that in other areas of life, even things like entertainment.
How many times you see people bitching and whining about how EA or Activision are horrible companies who make horrible products.... and then they keep paying for those products?
They cry about the lack of good games with fresh ideas but then refuse to look for them.
They're comfortable where they are and that comfort is enough to consume what's right in front of them instead of putting in effort to find something better.
Effort isn't comfortable, effort is hard.