Because those people, even if you don't know them, are part of the community of the building, and for the most part people will respect the building they live i

Because those people, even if you don't know them, are part of the community of the building, and for the most part people will respect the building they live in (obviously there are those people who don't as well). When you have strangers coming and going constantly, you lose some security in who is around you and when - and while it might be that the largest percentage of AirBNBers will treat the property with respect (and not just the apartment, but the building), it still increases the overall risk (either directly form bad short-term tenants, or even not properly closing the lobby door, or letting people in behind them that should not be in the building).