Sorry to burst your tech l337 bubble, but there are a lot of numbers in the phone book that aren't online, or if they are, would take far longer to find online than in a phone book conveniently sitting next to the phone.
Personally, I probably use the computer 75% of the time, but that's because mine is ALREADY ON. I'd bet that the vast majority of homes do not keep their computers on 24/7. If I'm downstairs in our house, rather than near my computer which is upstairs, it would be retarded to go upstairs to go online, rather than picking up the phone book that's already NEXT TO THE PHONE. Also, I'll bet the vast majority of people are, in terms of computer saviness, morons. Thus, their computers probably take a couple of minutes to boot up due to all the spyware and other crap that they have installed. Add onto that the time it takes to connect to the internet if they have dial-up, and you're talking about some serious time, compared to the 30 seconds max it takes looking in the phone book.
Oh yeah, and if these forums are any indication, there are too many people who lack the intelligence to conduct even the simplest of searches on the internet. How they found these forums is beyond me. Seriously. There's even some moron posting a when did you join Anandtech thread - apparently too stupid to even think of looking under everyone's names in any other thread for their joined date.