Because Germany used to be different; if you had a long-term contract from 90s that e.g. stated your rent was 300 EUR/month, there was very limited room for any increases (basically only when your house went through some kind of renovation or the energy etc. costs rose significantly). So somebody who lives long-term in that neighborhood pays e.g. 400 EUR/month, whereas new tenants pay 1500EUR/month for the same. It's pretty normal in the US to see these kinds of yearly increases, but not in EU outside UK and EEU.
Most Finnish rent contracts have a clause that allows the renter to adjust (i.e. increase) the rent yearly by a given percentage (usually according to a cost of living index).
