People do it because it is quick and easy. If Google is your homepage, then when you start typing stuff in, it goes into the search box and not the address bar. Typing 'facebook' and pressing return is pretty damn quick and saves some of the clicking and mouse moving. I know lots of people who do this. My parents don't really understand the address bar, for example.

As a side note, the author's arrogance is irritating. He can't think of a reason why people do something, so he declares it 'irrational', 'dumb', 'stupid' and 'highly illogical'. Perhaps he should have paused and considered that there could have been a reason behind the actions of so many people.