Rebecca Greenfield spoke to linguist Mark Liberman about why the verbal gaffe is so common:

The syntactic category rule means that when two words are confused for one another the "target" (the word replaced) and the substituting word are almost always of the same syntactic category. In normal speak: nouns replace nouns, verbs replace verbs, and so on. If "Obama" were a verb instead of a noun (as in, the Democrats are going to Obama the GOP in 2012), we would be substantially less likely to confuse it with the noun "Osama."