>You can't build them except in the places they already are.
I.e. in all big cities? So are you really arguing that the rent in SF goes up because you cannot build a residential tower in Redding, CA?
> Converting existing high density housing to even higher density housing is prohibitively expensive.
You don't need to convert. You can change zoning on existing lots, it happens all the time.
>Just build more housing at the market rate as long as the market rate is above your construction costs.
But it is not in most places. This is why developers are trying to get into prestigious areas where it's still possible. Nobody prevents you from building a residential tower in the East LA, you just won't be able to sell it. But yes, nobody will let you to build one in Malibu, where you'd be able to move units.
The issue is that as soon as you filled Malibu with commie blocks the area won't be prestigious anymore so somebody will be left holding the bag, developers are desperate enough to enter this game anyways.
> If it costs $300 to build a square foot
Just read the message, I've explained what.