Every large booking platform charges a guest service fee. It is calculated on top of the nightly rate the operator set, and it is the single biggest reason the same house costs more on a marketplace than it does on the operator's own site.
Booking direct removes that layer. The rate you see is the rate the people who care for the house decided on, and the money goes to them rather than being split with an intermediary. On a week-long stay the difference is often enough to cover an extra night.
There is a service argument too. When you book direct, your message goes to the person who holds the keys — not to a support queue that has to relay it. Special requests, early arrivals and questions about the road up to the house all get answered by someone who has actually stood in the driveway.