That's not exactly the fault of booking.com though. Hotels have learned that if they want people to book directly with them (which gets them more money), they need to make it attractive to customers. They can't really offer a lower price than booking.com and hotels.com due to contracts they have with those companies so instead they offer a reward scheme and exclude bookings not made directly with them.

They also sometimes offer lower "member only" price for members of their reward scheme as a way to get around their contract (in that case the lower price they provide than booking.com is not public but only restricted to members so it's not a breach of contract).

That said, I don't use booking.com, I find their dark patterns too deceptive for me.