I'm sure there are details I don't know, but naively it seems like enforcing those same valuation consequences for empty units too would incentivize filling units.
On a 30 year model, small fluctuations have big consequences, and they cascade even further if there are multiple units.
Not filling at all should too though.
If you discount it to 80% today and then rent it at that rate for 10 years, you'll lose 10% compared to if you leave it empty for a year and rent it for 9 years at 100% of asking price. Obviously financial calculations have a lot more complexity than that, but it illustrates why someone would prefer a vacant property. It's based on expectations of future income.
The fundamental problem is everyone wants in on the big money. No one really wants to serve the lower end. And the lower end would be upper middle class in some cases in a lot of other cities. Definitely all of it will concert to make parts of NYC the exclusive domain of the wealthy soon. (Actually, parts already are, I guess I mean more parts.)