[seanmcdirmid]

There is a lot of housing that is unimproved because the owners can't afford to improve it. E.g. you bought a long time ago, or it is used as low income housing now (conversely, in my neighborhood at least, the low income housing is older and less dense than the shiny new dense luxury apartments). The only recourse for that land is to convert it into a tower of new luxury apartments, gentrifying and pushing out whatever poor folks are housed in it now.

There are other ways to game this, there are always arbitrage opportunities when a tax isn't aligned with its expenses.