Exercise. People often forget (or aren't even aware) that output isn't the only factor in getting in shape, that walking is better for you than running, that the body just wasn't made by the forces at play for a lot of continuous rigorous activity (think the paleo diet logic but for exercise; in fact, if anything, the logic is applicable TO exercise), that gyms cost thousands of euro/dollars, and that many people just don't have time for it.
The icing on the cake is when you have no friends and someone says "the gym is a great place to meet friends", unaware that the problem is you're not good at holding a conversation and that that part about gyms being better for making friends is not even true. It's like, I don't know, gyms are trying to cash in on peoples' random life problems.