LAtenfour is right that a lot of girls don't have an internet connection.  Some of those girls tend to be a little on the nomadic side.  Others simply choose, usually for reasons of discretion, not to advertise on the internet.

Wantats has hit on the true value contained in the unknown, i.e., diamonds in the rough.  There is a certain excitement and anticipation created for some of us, myself included, by going on an uncharted adventure with an unknown girl who only advertises in the paper.  There is, of course, more risk involved than there is when going on a date with a well known and well reviewed girl.  It can be harrowing at times.  

However, there is far more reward as well, IMHO.  My three ATF's were found this way, none reviewed to date.  I still see two, neither advertise anymore, and the third has moved to NY.  Both have jobs outside "the life", and give new meaning to the term GFE.  I'm sure there are others of my brothers who have their own hidden gems that most of the rest of us know nothing about.

Anyone care to fess up?  Details, of course, not required.