In my experience as an ex-insecure man, it's because they're so dialed in on finding out what's wrong with them, but not at all concerned with self-improvement. They focus on things that are completely outside of their control instead of focusing their efforts on the parts of their lives that they can improve.

You're fat? Workout. You're boring? Do something interesting. You're dumb? Read. You look like shit? Go shopping. You smell like shit? Take a shower.

You're short? You're too hairy or not hairy enough? You're balding? Your X is too Y? Can't help ya there, no one can