Playing devils advocate here, but someone can immediately follow up their OW cert with an AOW cert, perhaps even in some aquarium like tropical location such that the second cert card means very little. I think being capable of free ascending from deep, shooting a SMB at a safety stop is something learned with dives and time in the water, more so than an additional cert card with minimal real reps.
But I agree that logic has little to do with it, and it's most likely all about liability. The box is checked for both the insurance company and the dive op more simply with a cert card requirement vs a real assessement of a diver's readiness for a dive.