GitHub's fork management is lacking, but as it is now, forks are a symbol of popularity, so authors have no incentive to prune them. Plus it would be like saying "you're not a real contributor" to fans (which, while probably true, is still rude and not worth being a jerk about).
A simple first step would be to hide forks publicly that don't have any commits since they were forked. But I think some people fork things because they think it's like adding them to their resume (I can't understand it) and they wouldn't be happy.
I sometimes wonder why people fork my repos but don't make any modifications. My feeling is similar to yours, since I sometimes asked the job candidates during interview to show me some of their open source, and they showed cloned repos, but I would not find their commits there.
I'd only add changes for ... legitimate users: if I forked but forgot about it, just simplify my life and remove this repo automatically, if I requested so.
Plus if I want, I'd like to track changes in forks too.
Depending on the repo it is - mirroring. When I feel the owner might be pressured into removing their repo.
I have a thicker spine and can hide behind a veil of public anonymity. For example, if someone tells me to pull my forked repo because it offends them, they'll get a bird and told to sod off.
There's been quite a few actually useful projects that have been pressured into shutting down - leaving only the forks to remain.
A simple first step would be to hide forks publicly that don't have any commits since they were forked. But I think some people fork things because they think it's like adding them to their resume (I can't understand it) and they wouldn't be happy.
The entire concept of "forked from X" is broken.