Think this one through for a moment.
Although the Adventure Diver course does exists, it does not even merit a place in the table of contents of the PADI instructor manual. It is a subset of the AOW course. It requires the student to complete 3 adventure dives and the AOW knowledge reviews for those dives. AOW requires only 2 more adventure dives, including the deep dive and the navigation dive, and the knowledge reviews. Why on earth would anyone stop and get the Adventure Diver certification when it only takes 2 more dives to get the AOW? The answer is simple--age. Junior divers cannot do the deep dive. If they get the Adventure rating, though, they can go on and get Rescue Diver, which would otherwise be forbidden to them. So there will be a very tiny subset of people who will have a good reason to get the Adventure Diver certification.
Once those people are old enough, they people can indeed go on and get the Deep Diver Certification without getting AOW. If they were to do it (and I wonder if anyone ever has), they can count the first dive toward the AOW certification. They would need one blinkin' dive after that to get AOW. Somehow in the next year, they would have to find the time to do one knowledge review and one dive.
How often do you think think that has emerged as a problem?