Presumably the implication is that it's because lots of homes are being bought by real estate companies as long term investments that they may or may not even rent back out. This extra demand beyond people buying housing to, you know, live in it, drives up the price.
A different tax policy could disincentize this behavior, leading to lower demand and thus lower prices.
That's my current understanding of that perspective at least.
A 100% tax on property for those owning more than 50 properties would cause a large movement in the market. But people will say what about the value being tanked when all those houses go on the market? I say, good. The value of homes should go down. People need a place to live. Houses should not be considered an asset. You don't treat your home like an asset. You don't buy it when the market is right or sell it when it's right. You don't improve it in ways that are most likely to increase value. You live in your house. It's your place to be. Whether it's a flat, a house, or a boat in a canal, it's where you live and feel safe and spend a large chunk of time.
A tax on owning too many properties would be subject to gaming. Many large companies already hold properties in tranches of 5-10m worth of properties per limited partnership, so the tax policy needs to be written carefully to disincentivize holding empty properties in a way that can't easily be gamed.
I think the tax policy is prop 13, which is a handout to rich homewoners that means they will never sell their houses (very nice for them, not so for anyone trying to get new housing)
