I do floss, but I genuinely don't see that this is obvious. You can do a lot of damage with mechanical force, to both teeth and gums! Starting a flossing regimen after not having one tends to cause pain--isn't that a signal to stop? etc.

Furthermore, correlation is not causation and it could well be the case that flossing is associated with better outcomes without causing it. For example, people who can afford to go to the dentist regularly are therefore regularly told to floss. People who care about dental health in general probably floss more, but also may be doing other things, consciously or unconsciously, to improve outcomes. Gut (and perhaps mouth) bacteria have behavioral effects; perhaps flossing is caused by having healthy mouth bacteria!

(at least one study says mouthwash is better than floss. That seems obvious to me! liquids are smaller than floss.)