as Geoff said, Viator & Tripadvisor and the ilk are booking agents, & they operate by taking commission on the bookings they take for the tour operators. They offer some guarantees about making it back onboard, or catching up to ship, or for refunds on not making it to the port, which may be comforting.
In NZ we have used Zealandia, Shore Trips & Tours & in Dunedin Tourworks, for our tours.