I find that German law idiotic. Of course it costs the landlord something to hire a broker, even if the renter is supposed to pay the fee. It still eats into the overall budget of the renter, that could otherwise have gone to the landlord.
You can find it idiotic, but finding a tenant is quite easy in popular places, so instead of finding a way to increase the overall price (which is also regulated), the landlords often just do it themself. In unpopular places, it is hard to increase the price.