"diet" implies a temporary change to achieve a certain goal. However, "changing your diet" has long term permanence in it. But this is hard for a lot of people because they don't have a fixed diet to begin with, instead just eating whatever whenever they're hungry. Same with exercise, people need to make that into a habit.

But forming habits / making lifestyle changes is hard. And when people hear they can just take an injection instead of make lifestyle changes they're like oo, easy!