Because it's a mostly unregulated market. A new competitor has minimal government regulations to satisfy in order to start and everyone and their dog can start doing this job (and believe me, I've met some interesting agents when looking for the best deals both looking for rent and renting out my property). The big centralised agencies don't really have a way to stop smaller agents offering cheaper services and serving different markets (even if they're just different neighbourhoods) is hard.

This is how capitalism works when there is no government helping monopolies to form in the name of security / justice / anti-money laundering / insert-politician-buzzword-of-the-day.