Why do people search for "Yahoo" or "yahoo.com" (or, for that matter, things such as "www.espn.com") on Google?
Raymond Chen has a look at this question. Summary:
A lot of people simply don't care to learn the difference between the search box and the address bar. "If I type what I want into this box here, I sometimes get a strange error message. But if I type it into that box there, then I get what I want. Therefore, I'll use that box there for everything."
I didn't know people did that! But then, I'm a propeller-head nerdy geek, not Joe Random User.