Actually a lot are... but that is more a societal issue than a "forum" issue. I find that on several (in fact, most) fora that I am active on, participation has dropped for whatever reason (a couple of examples, a Tundra forum, a black powder muzzle loader forum, one of the larger astronomy forum, several hunting fora).Some are, perhaps. But, I'm observing a significant decline in the "big tech" social media sites.
Part of the issue is that a small sect of members get tired of "pushing" all the content providing context/answers. That can happen even on social media (think FB groups). It just happens that it is not as noticeable as you have one interface to that site, so more people get channeled into it. I've noticed it even in the the Winnebago FB group I participate in.