They have been in a slow decline, along with unions, middle class, and manufacturing, during the rise of the internet and globalization.

Actually, they've seen an uptick over the past decade. Specifically because of the need the OP implies, and younger people becoming aware of their existence. The elks, for instance, had ~800k members back in 2012ish, and are over a million currently.

Which is to also say that I think these clubs tended to skew towards a white middle-class without a college education. That is a declining demographic.

What about the Insane Juggalo Nation? They seem fairly active still.

I think a big part of the problem is that most of them have a purpose other than just socializing, that not everyone is necessarily interested in.

At least some of them also have fairly high dues. I remember looking into Rotary in my area about 15 years ago, and just feeling like there was no way I could justify paying however much it was per month, even knowing that a lot of Rotary's purpose is to try to do good works, given what I was making at the time. (I don't remember how much it was, but my impression, at least, was that even if I was interested in actually devoting my time to it, it would still be more expensive than I could comfortably afford on a just-barely-above-national-median salary.)

And there's also a religious aspect: the Knights of Columbus are explicitly Catholic; while it's not officially religious, my experience with Rotary was that it had a very high overlap with churchgoers, and my recollection is that they opened and closed their meetings with a prayer.

And while the VFW may not require members to be former military, it's never going to attract large numbers of people who aren't, just because that's its stated purpose, baked into its very name.

I don't think those kinds of organizations will do as well with the younger generations, today and into the future, given these things. Now, if there were more interest-based clubs/groups around (like community singing groups, tabletop gaming, nature trail groups, etc, just to give a few examples that would interest me), that seems more likely to appeal.

I just looked at Rotary Club dues, $36 for 6 months (6 bucks a month). Doesn't seem too bad.

For interest-based groups, I'm constantly seeing large bicycle groups cruising around, so such groups do exist (at least that is one that is openly visible). I periodically will jump in on Linux user group (or similar) meetings in my area too. But they typically have about half a dozen people that show up. I know that hackerspaces were popular in some areas for a while, they may pick up again post-pandemic.

That's definitely much lower than I remember it being back when I looked. (Which doesn't necessarily mean that it is lower—it's not something I took special note of.) Still, for me, not something I think it worth my time & money to do, but not nearly as much of a pinch as I was thinking for someone that does actually like that sort of thing.

I can definitely get on board with that sentiment. A number of years ago I was all set to join a Hackerspace in my area, but for two things. One was the monthly dues, $50 a month. I was already budgeting out to the last dollar of income, so that wouldn't have worked. Second was the commute into the city to make use of it. If I worked / lived in the city it would be a bit different, but just as there was the issue of budgeting money I had just as much of a problem budgeting time for it. It really does seem to me like we have less time available then what I recall my parent's generation having (for example my Dad always worked on family cars despite working a factory job with a 60 minute daily commute). And they still had time to watch 2 - 3 hours of TV in the evenings.

Edit: I just looked again, that is the dues that the central organization charges each chapter for chapter members. The individual chapter dues may be much higher, and probably varies depending on location.

Many of them are affiliated with some organized religion which has been waning for a while now.