Maybe because finding a decent landlord to rent from privately is not easy as easy as going to an agent.
Agent fees should be banned simple as that, if you are a renter, with the up and coming general election this is a good time to challenge them on this issue.
As a landlord I avoid using agents at all - absolute sharks for both the tenant and the landlord. Going through an agent doesn't protect you against having a dodgy landlord - it just costs you more. I personally rate Letting Agents lower than estate agents, lawyers and recruitment agents.