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Because he doesn’t lose anything, but has the possibility of gaining something.

He's definitely losing something. Property taxes are based on area and number of rooms. They could have found a tenant looking for the right number of rooms in their budget.

Unused capacity might be claimed as a business loss?

Or he advertised 1, 2, and 3 BR and took the first that came along.

You are assuming demand is equal for all of those. It might be in some places, but not others.

A bit weird but there's at least an argument that there's some wear and tear associated with using the rooms and, presumably, the landlord thought that the slightly cheaper/smaller apartment--with a possible upsell--was more rentable.

If the rooms aren't used, they won't experience any wear and tear and they won't have to be cleaned again for the next renters.