I used to think that but after looking a bit more closely found that the inventory and usage is quite different. VRBO users typically want an empty house set up for renting (i.e., not even any personal belongings except for necessities). With AirBnB, even the "Full Home" options are frequently/typically someone's house, including their belongings. And of course you would never see a share on VRBO.

It's actually sort of hard to find vacation rentals on AirBnB. There just aren't that many of them compared to VRBO and Flipkey.