Cause they can't (exercise). Some people just don't have the willpower to actually exert effort. I almost can't blame them, since it seems like a hereditary disability for most, like being born blind or something.
Yup, many disorders drain you willpower/energy.
I have one, I know what it is like and it's very difficult to fight.
But no way I would willfully have a tapeworm.
All the things I really like to do are either illegal, immoral, or fattening. And then: Golf.