Because booking.com answers the question "where can I stay given place, date, and budget"? While direct access can only help you when you already know where are you going. The latter is fine if you travel to the same place many times, the former is more universal. As for reliability, I dunno, I used to travel a lot, and in my experience troubles mostly come with a particular destination, not with the method of booking. I.e. if a hotel is unreliable (poor business culture, or overbooking, or whatever), it's unrealiable.