Cannot is the only negative form that contains not rather than -n't. Theoretically, since it is a single word, you can say why cannot you... without a problem. My theory is that modern English speakers don't want to put cannot before the subject because it contains not and sounds like can not.
-1 Saying that something "cannot be overstated" is exaggeration and clearly never true, because anything can be overstated. (You just keep repeating that thing to the exclusion of everything else: now it is overstated.) So is using this phrase sloppy writing, or is it a well-understood phase that no longer means what it appears to mean?
As does Thesaurus.com: The terms cannot and can not are identical in meaning and are typically considered to be alternate forms of one another. However, the form cannot is much more commonly used than can not. Cannot has become the standard form in formal writing (and is typically the form recommended by most grammar resources and style guides).