There are market dynamics at play in both places. If the rents are increasing by double digit percentages within a couple years then that means that demand is

There are market dynamics at play in both places. If the rents are increasing by double digit percentages within a couple years then that means that demand is rapidly outstripping supply. There's really only two options here: Reduce demand by banning people from moving to the city, or increase supply by building a lot more housing. #1 is illegal in the US and likely illegal in Germany too, so #2 it has to be.