> Why does it have to be fair to the landlord?

Well, it doesn't have to be.

But prospective landlords tend not to build new units (or maintain old ones) when they don't feel they are getting a fair deal. It leads directly to the situation they have in San Francisco, where property owners only rent to family, coalesce many smaller units in to few larger ones and sometimes withdraw their units from the market entirely.

Landlords are investing capital and their own labor in their properties. If the expected return on that investment is less than what they could get from government bonds and a 9-5 job, don't plan on a big expansion in housing stock.