You buy apartments and charge people significant part of their earnings for the privilege of having a roof over their heads.
Why is that bad?
As an owner you don't live there yourself so you only have an incentive to keep the place in shape good enough to be rented. Which is usually pretty low.
The worst part is that if you can't find the people to occupy your apartment your costs are pretty low (you don't have a profit but that's not the same as money being drained from your account). So you can buy and hold apartment for years at very low cost. This creates artificial scarcity (in addition to actual scarcity) of places to live that keeps the prices up.
I think progressive property tax would enliven real estate trade. Prices would drop and more ordinary people could afford to buy one on credit instead of renting. This could save working people a lot of money so they can actually start their lives instead of slaving on their first job with just enough money to pay the rent.
Tax should be constructed in such way that holding even large apartment, or even two or three would pose no burden or almost no burden, but you might be better of selling your fifth apartment even at 50% below market price than holding it for a year.