As far as I can just looking around my neighborhood and looking at a few known-vacant buildings, there are plenty of houses that are vacant for investment reaso

As far as I can just looking around my neighborhood and looking at a few known-vacant buildings, there are plenty of houses that are vacant for investment reasons. And the dynamics of the market sort of reinforce this. Real estate is a great investment but land lording is hard. Renting below market lowers the value your investment and so does leaving your investment open as a rental indefinitely. Sure, just about anything that opens up will get many applications. But most of those will be from people who technically can't afford the location - because house is unaffordable, people making 3x the rent you are charging are scarce and the most desperate people put in the most applications, so any landlord will find most people applying look like bad renters and not want to deal with the situation. And for the same reason, some portion of the "between renters" category may be actually off-market too. If the landlord is for the perfect renter, they may look forever.