Bug Catchers are a weird breed of Pokemon trainer - they've specifically chosen some of the weakest Pokemon to be their specialty, and often don't evolve them beyond their worst form. Hell, lots of them have, like, 2 Metapods that can't even attack.
Although, really, any Pokemon trainer that specializes in one TYPE of Pokemon is kinda asking for trouble, since another Pokemon with a type advantage can absolutely decimate their lineup without breaking a sweat. Really, that trouble they're asking for is so bad, they should make it double.
Sorry Caterpie, your life is going to be terrible forever, and there's nothing you can do about it.