I edit books for a living, and typos are embarrassing but they happen for all sorts of reasons. Sometimes it's a case of "readers see the one thing I failed to correct and not the 3000 things I did correct". Sometimes a designer will make a last-minute tweak and forget to show the page to the editor for checking. Sometimes someone at the printers will accidentally use an old PDF file for the final print run and not a newer, corrected version, and there won't be time or money to reprint. Frankly, it's amazing that it doesn't happen more often!