Well, they don't, but it may not matter. Most people in Europe who have great teeth don't floss. They think it is humorous that we pull string through our teeth the same way European women wonder why their babies don't have more birth defects if a glass of wine causes fetal alcohol syndrome.
America is the home of epidemiology, even if Europe politicizes it more when it comes to science. With 9,000 epidemiology papers published per month, and corporate media twerking sane science and health policy by promoting anything that will scare people or create a diet fad, it is a wonder there is anything that doesn't cause cancer.(1)
The problem with flossing epidemiology is the same as all epidemiology - people lie on surveys. You know this if you follow politics. One election since the modern polling era has been correct; 2012. When President Obama was such a sure lock to win that a European betting agency was just as accurate as Nate Silver on how many electoral votes he'd get.(2) In every other election - 2016 is a great example, since after their candidate won in 2012 a whole lot of academics declared polling 'science' - the results would get you fired at any private sector company where accuracy matters.
In epidemiology, there is no accuracy. There is only statistical significance. And that is so easy to attain that you can declare coin flips prejudice against heads. Or tails. Using the same data. It is certainly easy given enough foods or chemicals and enough diseases or healthy benefits to claim statistical significance.
So companies that make floss certainly promote anything that 'suggests' a benefit, and dentists will happily offer up their anecdotes because it gives dental hygienists something to lecture patients about, but those are just-so stories. Like claims that liberals have prettier daughters or zebras evolved stripes to confuse flies.
If the dog craps near my outdoor furniture to protect me from animals and I don't die in an animal attack, are they wrong? If you see the flaw in that, but believe in acupuncture and the homeopathy of 'endocrine disrupting' chemicals, I know how you vote. And it is not for Team Science.
You may have healthy teeth, and you may floss, but the floss could be as relevant to your health as music therapy is to a cancer patient getting chemotherapy. It is a fine adjunct if it makes you feel better, but it will never pass clinical trials on its own.
Flossing is easily replaced by chewing gum or brushing your teeth twice a day - if you get dental check-ups once per year. If you floss and never go to the dentist, you are going to have bad teeth unless you hit the genetic powerball when you were born. That is how you not the flossing may not matter. Genetics and any hygiene matter, flossing may be a placebo. Like solar power is for the 0.1%.
The president of the American Academy of Periodontology conceded there is no evidence, but since there's no harm in doing it to keep doing it.
Except that is the 'Buy Organic' marketing tactic and there is harm. If you shame poor people by telling them they are doing a disservice to their children not flossing, you are simply unethical, because that means they can't spend money on something that is helping their families. Dentists should be better than that.
There are lots of substitutes for flossing if you have something stuck in your teeth, like gum mentioned before or a toothpick. But if you have the money and time, go ahead and run string through your teeth. Just be sure to do it back and forth and not up and down the way most do - that is definitely more harm than good.
(1) Or, in the case of California and its fetish for warning labels on anything Europeans 'suggest' may cause cancer in high doses while American epidemiologists claim a benefit, sometimes both causing and preventing disease. Like coffee, which was required by Prop 65 to be banned except politicians suddenly decided to waive the ban. And IARC then lowered the warning on coffee so California would not have to ban it. This is more common than you think. The Biden FDA recently said red dye no. 3 was unsafe using only rat models - so no human relevance - but it happens to be an election year and the administration doesn't want to have to sue California for banning products illegally.
He needn't worry, California has not voted for a Republican this century and since taxes and regulations sent a whole Congressional seat of Republicans to other states, that is not changing any time soon.
(2) If you believed polls in 1992, 1994, 1996, 1998, 2000, 2002, 2004, 2006, 2008, 2010, 2014, 2016, 2020, and 2022, though, you were disappointed.