On any book I’m editing, and that any editor is editing, it gets read multiple times in multiple forms. So not just the script, but the art as it comes in, the lettering at least twice–once when it’s lettered in B 7 W, and once when it’s composited together with the coloring. It also goes through our proofreaders twice and past another editor for “read-out.” We likely read it more than you do.
But here’s the thing you have to understand: nobody is working on just that one issue. In a given day, there are dozens of issues all going through that same system, and hundreds of corrections that need to be made and made correctly. Anybody could be focused on one single comic book and stand a good chance of making it pretty good (though even there, mistakes would happen), but we’re dealing with a whole line of comics every month.
For every mistake that makes it through, there are thousands of other things that are caught and fixed and changed. But you never get to see those.