Why are so many Airbnbs backyard guesthouses, ADUs or converted guest suites?

I’ve been looking through Airbnb and vacation rentals on the Coastside and noticed how many smaller 1-bedroom/studio listings are guest suites, backyard guesthouses, in-law units, above-garage spaces, or require access around/behind the main house.
Is there a local zoning/housing reason this setup is so common around Half Moon Bay? And for people who have stayed in HMB, did you care whether your Airbnb was a detached/front-facing space versus a backyard unit, converted portion of a house, or above a garage?
Mostly curious because Airbnb’s “entire place” categories can make some very different physical setups look similar in search results.