The problem is if they have some renters at the high price it is better to say they have tenants coming (even though they never arrive) than rent for cheaper and risk the old tenants find out the price is lower and they demand a reduced rate. Now add in a bank who is auditing your books: if you have some empty units "almost rented" that looks better than full units but you are not making enough on rent to pay the mortgage - even though either way you are not making enough in rent to pay the bills.

Banks don't conduct such audits.

If the loan requires that all units be rented for $x they will do something to check.

Bank loans to landlords do not contain convenants requiring that all units be rented for $x. You appear to be confusing policies that some lenders apply when originating loans with covenants that borrowers have to follow on an ongoing basis. Those are not the same.

> then deducting paying the loan rate/interest over the time is more profitable.

What does this have to do with not renting the apartment? Any business will deduct interest payments as a business expense.

And a vacancy tax isn't a good idea either. (Though it's better than the UK's tax rebate for vacancy.)

Just use a land value tax.