Have you seen the GameFaqs message boards lately? A decade ago, it was sprawling with activity. A popular game could easily get 30 thread refreshes per hour. Nowadays, they're pretty much dead. Even NeoGAF had a steady year-on-year loss of traffic, well before it went down.

Reddit, Steam Forums, Twitch and Discord is where the youngcrowd talks about games, and there are some VERY active DCS Discords out there, with 20 to 30 posts an hour easily. Subsim.com is also still going strong, with a high "refresh" rate of people. But that is mostly due to the incredible mods that are there, and people having trouble with getting it all set up.

However, I do see a common tendency among all forums I visit, and not only the gaming ones: People that belong to the core of any forums these days are all 40+ years of age, have been on that forum for many years, and that forum is one of their main hobbies and pillars of online social activity.

The "why" is quite simple in my view.
Before Facebook, Youtube and Steam broke through to mainstream a decade ago, the main hubs of online social activity were forums and instant messaging programs like MSN and ICQ. The latter being mostly used (at least in my environment) by people you already knew, and forums were used for people to talk about shared interests. However, the hobbies and interests of young people change overtime, and so do the crowds they visit. Adults which are settled come from work and check on their family and friends. They're not really looking for a new crowd because they're happy where they are. They have a place where they can talk about their hobby, know the people on the board for a long time and have a good time along the way. The social relations of young people tend to fade away quite quickly while they are still developing themselves into adulthood, and they are much quicker to pick up new things like Facebook, Steam, Instagram, Discord, Twitch, etc... So all the new influx of simmers go the new media, while the adolescent simmers have mostly changed where they post for news and chat, and keep forums around for mods and other archival stuff.

So while all the younger gamers have transferred over to those new media, the message boards and gaming sites are mostly visited by the ol' captains. This is just how things go, just how mailed letters are the stuff of 80-year olds, while the younger lot uses e-mail.

And fun fact:
Sim games attract the same amount of crowd as they always have, according to multiple sim devs. It is just that the expected standards have grown, which pressure profit margins. Thats why fewer games come out, even though the cake is as large as ever (or even larger, with the rise of the middle-class in asia and all that.)