There are places on Airbnb that have never been rented. How would those places be verified, which is necessary if you want to say "Every listing is verified", if you do anything other than send employees to those places?

Craigslist, Booking etc don't make any claim that all places are verified so they don't need employees to do that work. If Airbnb are going to make that claim then they will need people to do the verifying.