Because there are massive positive externalities to people stably investing their communities (often called "social capital"). Your attitude of putting your head in the sand about things you can't measure precisely is the consistent folly of the High Modernist, "systems are simple so we can tear them down and remake them" line of thought that destroyed the American city.
I actually think cities tend to overregulate Airbnb (and housing construction), but if your model of housing policy can't tell the difference between a tourist and a resident, it's woefully incomplete.
The thing is that a NIMBY attitude does absolutely nothing to create more social capital. Places where social capital is highest are often precisely those with the strongest traditions of welcoming hospitality towards short-term-visiting ("touristy") outsiders.
I agree, and I'm probably closer to you in my opinion of nimbys than you think. But I don't think there's a problem with being aware, at a policy level, of the distinction between residents and tourists.
I actually think cities tend to overregulate Airbnb (and housing construction), but if your model of housing policy can't tell the difference between a tourist and a resident, it's woefully incomplete.