Realtors enjoy lots of laws that protect them from competition. They have considerable market power. See this DOJ website:
http://www.justice.gov/atr/public/real_estate/commission-tab...I recently bought a house and was amazed by the fees. My buyer broker basically opened doors (I don't mean figuratively, I mean he literally took keys out of lock boxes and opened the doors). I plan to do a FSBO when it comes time to move.
Zillow and RedFin are perhaps changing the industry some. I think most people go with realtors because they are intimidated by the process and it's such a large purchase. I will be much more comfortable handling it myself the next time around.