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.
The natural end condition of an unregulated market is a monopoly. That is why we need regulation.
So, something is operating to provide implicit regulation. In this case it seems like chaos serves that role. Monopoly relies on a self-ordering property common to most market conditions.
The explanation GP provided is extremely low cost of entry. An agent doesn't even need an office. Here is my country many agents go free wheeling with a phone number and a bike. All they need is some connections with homeowners and they're off to business.
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.