I'd like to further comment on rattler's observation--which is spot on.
A lot of people CLAIM to be proofreaders, but many are really frustrated writers or editors. As a result they spend more time changing the prose rather than looking for typos or other errors. Real proofreading is a skill almost totally different than writing or copy editing, and not many people have the mindset to do it right, since it can be pretty darn boring. Which is probably why there's always more demand for GOOD proofreaders than for writers! The pay can be pretty good if you have the skills. I know a woman who makes a very good living proofreading scientific articles.
