Because I've run across a few IP editors that seemed to care, even if they don't edit on a consistent basis.
These type of editors are a pleasure to come across. I suspect more than a few are current or former editors who can't help but fix an error they come across when browsing an article. Sadly, they are very, very rare. So no, not "most"; more like "virtually all". Like David said: if they care that much, they will usually create an account.
AGK