Licenses aren't just for ensuring competence (as in the case of driver's licenses). Sometimes, they're to make sure that the government knows about the people doing it (as in the case of car registrations), either to collect tax or to be able to find people associated with an activity. And sometimes they exist to limit the number of people who can do something (as in the case of liquor licenses in many areas).

In this case, it seems like SF wants to "license" rental activity in order to tax it, and it wants to tax rental activity in part to limit it. Zoning could do this, too, but a tax is more flexible on an individual basis.

This is especially meaningful in a city in the midst of an affordable housing crisis.

The license is not to ensure you know how to host. It is designed to limit the impact of hosting on neighbors and the housing supply. People didn't sign up to live next to a hotel when they move into a residential neighborhood.

The license is to rent out your place. Does renting your property on a short term basis suddenly release you from requirements like making sure you have heat and water in the unit or meet zoning regulations? There's and argument to be made that this level of regulation is uneeded or causes more problems than it solves, but the regulation is still there and AirBnB I'd doing there best to avoid having it affect their business

Plenty of things can go wrong with a place to sleep. Bedbugs, for example.