Because cavities exist, apparently, questioning the existence of dentists is equally as valid as questioning the existence of a supernatural being responsible existence itself. I suppose the video “Dentist Says God Doesn’t Exist” tries to address the problem of evil — why a god would permit evil or unwarranted suffering to exist. As it appears, the analogy is that cavities and other oral hygiene problems persist among patients despite the existence of dentists, so it should also be conceivable that evil exists could despite the best intention, or at the behest, of god.
While it’s true that patients have free will to ignore a dentist’s advice, just as they can ignore the teachings of any religion, the difference is that dentists are not supposedly all-powerful, all-knowing, and all-loving supernatural beings. Such a god would not only perfectly know people’s future decisions to commit evil but would have the full willingness and complete capability to prevent it, or at least certain kinds of evil or suffering. That isn’t the case for dentists or any (limited) natural being.
But even if dentists could perform some sort of hypnosis on patients to have them practice ideal dental hygiene, there would still be the need for dentists since they, unlike an all-knowing god, can’t know of or necessarily have the means of preventing sudden and accidental dental emergencies.
I’m afraid this video is a poor attempt to blind us from the senseless evil and pointless suffering around us.