Why do some people think it's fine to never work out if you're naturally thin?

Exercise does a hell of a lot more than just make you look good. I don't really think already looking good is a good enough excuse to not work out. I mean, it's their life and all but just because the outside looks like it's doing well doesn't mean their insides are going to be fine.
Why do some people think it's fine to never work out if you're naturally thin?
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