> I mean, if you owned an apartment that could rent for $3000/mo, then keeping it empty is a direct reduction of your income by $3000/mo.

Not realizing potential profit is not the same as losing money. If I own an apartment that costs $0 to maintain, then not renting out that apartment costs me nothing. It's not "cripplingly expensive" just because I _could_ rent it out for $3000/mo. It actually has zero impact on my income, and I could stay like that forever.

The cost of keeping an apartment empty is only the amount it costs to maintain it.