Eyebrows that are too skinny aren't always the result of overplucking. Plus, if they were, they would grow back and that person *should* have learnt their mistake.
Some people, especially with fair hair, have very thin or faint eyebrows. So there's pencils and stuff for helping fill them in. A good way of seeing this is to you tube *Pixiwoo* 2 sisters whom do various make up looks (like celebs, certain looks, showing off new makeup, etc). They have very faint eyebrows so they always have to fill them in. You see the difference.
However in the other extreme some people have very thick eyebrows and unibrows. Plucking and waxing helps take away the uni effect, and also to make their eyebrows neater and better shaped. As a girl you wouldn't wanna look like Charlie out of Busted (you may have to ****** who he is, and his eyebrows!).
Even though most girls do so much to their looks, doesn't always mean they are doing it ONLY to look more attractive to other people, or that they are unhappy with their looks. For example, I lightly pluck my eyebrows to make them slightly more arched, because I personally like them looking that way. Nor do I expect anyone to notice the small things I do to my looks, because it's my own personal novelty.
I don't hold it against men if they can't understand or appreciate it all.
p.s. and yes, some bad eyebrows will look like drawn on cartoon mickey mouse brows. That isn't the look that most girls aim for.