Because hospitality resembles nothing else, and generalist web studios learn that on your project.
A property does not sell a product. It sells dates that expire. An empty room on 12 July is never sold again — it does not go back into stock. That changes everything: what goes first on the page, where the button belongs, which season you build content for.
You are also not alone in your market. You are simultaneously a customer and a competitor of the platforms. Someone who has never read a platform contract does not know how to word a best-rate promise without landing you in trouble.
Then there is the technical side: booking engines, channel managers, overbooking, seasonal rates. That is not something you learn on one project. You learn it on twenty.