Because of competition. They can't all conspire together to price fix (or if they do they could be arrested).
I did wonder briefly if a land tax might decrease competition => higher prices, but then I recalled that the supply of land is fixed.
This SALT deduction is part of how they penalize landlords already, which is just passed on in the rent payments. This is part of the reason renting is more expensive than buying (comparing just monthly payments), in addition to management fees, profit, and repair/renovation reserves.