Don't need to. Look at the workspace of an experienced Mac user (who's using a large-ish display) and you will see that there are almost no maximized apps, and that plenty of different apps share the screenspace at once.
Beginner users (and non-Macheads) hate it, but I've yet to meet a switcher than didn't like it in the end.
It's a matter of paradigms - Windows doesn't really have a lot of inter-app interactions via the default UI, so there's no real need for multiple apps to be visit at once.