Why are sitter reviews not showing as a verified stay?

Because the sitter is new and hasn't booked any jobs on Rover. These are testimonials from personal friends, family or others they've worked for outside of Rover. All new sitters are encouraged to solicit testimonials to start them off on the site. When they do book a job on Rover, then that pet owner would be asked to provide a rating and a review. Only those would be designated as a verified stay.