A substantial amount of what I write consists of what I am doing right now: comments on articles, messages on message boards, answers on Quora etc. In terms of word count it easily amounts to a novel or two per year. But of course, it isn’t “real” writing because I don’t stand any chance of getting paid for it. :-)
Still, I find it does help to keep the creative juices flowing, and at least Quora regularly gives me idea for non-fiction articles; sometimes it is easy to adapt an answer there into an article for Medium.
So we probably write more than we think we do. But such writing doesn’t feel like work, whereas a “real” writing project easily does begin to feel that way. And that is probably why lots of very fluent writers end up producing little in the way of “real” work.