They give up because they don't want to change or put in effort.  They like the idea of being a victim of their circumstance so they can get perpetual pity from other people.  I mean their literal mentality is: Why be better than you are when you can just say "I tried and it didn't work I'm just naturally fat I guess", then everyone can pat you on the back for "trying" and reassure you that you're beautiful just as you are and you don't need to change for anyone, not even yourself.  These assholes are just lazy and they don't have the motivation to change because they don't feel ashamed of their shitty bodies anymore.