Berlin is a very odd city for renting. There are so many protections for renters. I have a friend who's parents bought a place there while she was doing her PhD. She got a roommate who was starting a PhD at the same time. This friend has since done a PostDoc in the US and worked nearly 5 years here (read: the original rate was set a while ago). Meanwhile, her old-roommate refuses to move out, pay more in rent or pay for utilities. Through a combination of ignoring messages (knowing that no one is in the country) and citing every renter protection one can, this person continues to pay only ~500 euro/month for rent + all utilities.
It doesn't help that her parents live in a different EU country and need to use Russian-German lawyers, who seem to be fine taking money even if they can't do much.
I imagine that cases like this have driven up the rent prices quite a bit.