my best guess is that it's because pets don't really have a choice. we know that we can and should eat sensibly and exercise regularly to keep ourselves healthy. your cat just knows that it's hungry, there is food in its bowl, and it has no reason to expend energy if there's nothing prompting it to do so, and it's incapable of connecting its eating habits and lack of exercise with its inability to groom itself or jump up onto the bed like it used to.

it's kind of like comparing boxing to dogfighting. they're both dangerous to the participants, but one activity is being entered into consensually with a full awareness of the risks, and the other is exploiting the instincts of an animal that doesn't know any better.