People's teeth (and brushing technique/effectiveness) are way too varied to make such a general statement. I flossed for the first time at some point in my 20s, did religiously for a year or so thereafter, and still do occasionally - but at no point was it like 'how on Earth have I not been doing this' or loads of gunk coming out, nevermind rotten and smelly. Maybe you have just the 'right' tooth gap that it's both too tight for your brushing but not so tight that stuff doesn't get stuck.

(And for whatever it's worth, all dental check-ups since some point in my early teens at the latest have just been a quick look around and yep all good - I'm not saying this from a position (touching wood I don't eat my hat next time of course) of terrible dental health.)