A houses value is not theoretical though. You own land and a roof to live under
But that doesn't mean you can turn a profit from it, or even break even. If you want to do that you have to sell it to someone, and there are multiple reasons why you might not be able to - maybe you spent too much money renovating it and now nobody wants to pay that much. Maybe a bunch of new housing was built and the value crashed. Maybe Detroit happened and the location and land it sits in is literally worthless and nobody wants to live there. - until you actually find a buyer for it all houses have only a theoretical value, as do all companies.