There are several reasons. First, it's more personal. You'll get a local technician to work with you one on one, instead of dropping off your electronics at some mega store and hoping for the best. Personal technicians can work on the problem with you, so you understand what is happening and feel comfortable that you aren't getting ripped off.
Second, many independent technicians do this work as a hobby. They want to be working on electronics.
Third, you're welcome to contact as many people as you feel necessary. This will give you multiple opinions on your particular problem and help prevent overpaying for the work.
Competition is good for all of us, our geeks get repeat customers because of the quality of their work and the fair prices. Mega stores get repeat customers because customers don't know where else to go.
LocateAGeek.com, now you've found us!