Because we need a low risk system to track whether people are net-contributing or -draining society's resources, otherwise it isn't easy to tell who is creating more wealth so they can be supported. Gold remains the best option after centuries (if not millennia) of experimentation.

> Tax what you want less of.

How do you apply this to housing?

You want investment in housing. You don’t want slumlords ramping up prices for slums. Presumably somewhere has got the balance correct. I haven’t been to that place.