They absolutely do and would do more often without copyright laws holding them back. Every person who finds a book they want to share with others would be free to make improvements as they saw fit, and that would certainty include edits and correcting errors.

Even today we have rouge editors who take copyrighted works, then illegally edit and distribute them along with their changes because they feel it improves the works in question. Examples include fan edits of films to improve the narrative and streamline edits of mythbusters to remove obnoxious repetitive cruft.

Other examples of free editors can be found in Wikipeida articles, fanfic projects, fan translation projects, and video game walkthroughs. Even mods/patches for video games can be a form of "free editing".

There are very few forms of media where I haven't seen people providing some form of editing services for free. (I can't say I've seen it for oil paintings yet I guess)

I think Wikipedia is a massive, living counterexample to this. It has tons of problems, but the lack of people willing to do the work for free is not one of them.