Exactly, it's an incentive game. Airbnb has an incentive to maintain high rankings across the board, because customers compare ratings across platforms as if they're equivalent. If they see a majority of 3/5 on Airbnb, they're just going to go on Booking.com and book a 4.5/5 hotel instead.

The only way to avoid this I can imagine is relying on cryptographic zero-knowledge proofs to defer ratings to an external network (ideally decentralized, but a neutral third-party ala TripAdvisor could work too). The platform gives the customer and provider a token when the transaction takes place, and they can use it to anonymously sign a review on the external ratings platform.