I believe the answer to that is the market segmentation. There are so many different type of segments (by programming language, years of experience, company size, industry etc.) that it is very hard for recruiters to spread across too many different segments. So they stick with a few, work them for years, build up a huge network and get really good in these few segments. But of course there is still lots of room in other segments for less qualitative focused recruiters to make a buck by just playing the numbers game.