The German constitution mandates that (property) ownership brings obligations for the public good. Ownership is a construct secured by society, so society as a whole must at least partially benefit. As such, keeping a building empty can be considered theft, too.
> As such, keeping a building empty can be considered theft, too.
Your assertion makes zero sense even if we accept your thesis at face value. For example, it's absurd to leap from "society mut at least partially benefit" from private property to assert that private property not being used in compliance to your personal taste to be theft. That's just plain nuts from any angle.
You’re absolutely welcome to make an argument how a building kept deliberately empty of tenants over years benefits society.
You can consider that position nuts, but given the amount of public support and legal backing for a current initiative in Berlin to enact eminent domain against entities that hold > 3000 flats, “plain nuts from any angle” seems to be slightly exaggerated.
There's more to life than property rights. Keeping buildings empty in an area where there's high demand for buildings is harmful, and real estate is inherently subject to regulation already. Opponents of squatting do indeed complain that it's theft of some sort and should be illegal, and that's hard to deny, but at the same time it's a very cheap and pragmatic solution to a problem: one way or another, it ensures that buildings are kept in use.
Most private property was at some point converted to private ownership by violent acquisition. Owning land is not a natural right -- of you're not using it for societal improvement it seems right that society can cease to recognise your right in that land.
A similar thing happens with IPR, whereby governments can coopt it in times of need.
