When you press review question/answer on any post on the review page there appears an edit link. That is, when you press review question/answer you don't end up at the page of the question/answer but the question/answer is expanded so that you may review it and act appropriately on it. So, since reviewing doesn't take you away from the review page I don't think there's reason to add an edit button.
Here's how it works. You look at posts in the review page and find one which you want to review
after you click it the posts expands and you can read it in full format.
As seen, there's an edit button at the bottom of the expanded post.
This functionality was newly added, see Allow marking posts as reviewed on /review and hide them for details.