Meta-analyses of RCTs do indicate flossing prevents gum disease, though, which is important. See another poster in this thread who suffered bone loss as a result of gum disease.

You're right though that there's no established effect on cavities.

Having said that, it's difficult to get people to actually floss, so there's a certain amount of uncertainty about the results of those RCTs, and whether participants were doing what they were supposed to be doing.