Edit: I'll even add: most american adults spend $100 per month, not per year, on frivolous stuff. Fast food. Cable TV. Unnecessary high-end phones (instead of mid-range phones). All these people could definitely afford $100/year.
I knew many people -- climbers, travelers, dirtbaggers, backpackers -- who really wanted to see the public lands that they partially own but could not afford to travel in luxury, people who ate ramen and rice and beans for months just to be able to afford to see the lands. There are also people who live in campers and vans because they can't afford rent. That $100 could definitely go to better uses.
If $100 is nothing to you, perhaps you're the kind of traveler this website is targeting. It's definitely not for everyone. I've met many amazing people on Couchsurfing and Warmshowers, which (at the time) were totally free and still today are much cheaper. The vast majority of the people I hosted were very poor but very well traveled, with stories to tell and friendships across the world. Very few of the rich people I've met have had the same experiences to share. Hosting them was a privilege, and not something I would've wanted to charge them for even if I could. I also met very interesting hosts traveling the same way, because at the end day doing that becomes way more relational and way less transactional -- the opposite of luxury travel.
FWIW, it is a genuine question, and depending on the answer could make the service actually very valuable. For example (only):
* If they manually vet each member not just with a basic ID check (ID.me etc.) but also with a background check and a Zoom interview, or otherwise improve community trust and relationships
* If they provide a built-in calendar/scheduling system and provide support for last-minute cancellations, rebookings, etc.
* If they provide vastly superior UX or support compared to Couchsurfing and Warmshowers
* If they provide support for international travelers, especially for accidents, medical issues, translations, whatever. But so far this seems to be within California only.
But from their website it's not clear that such value-adds are being done, so then the question is not whether $100/yr is affordable, it's what $100/yr gives you that $30/yr does not. If you're rich, that's not a big deal. If you're poor, that $70 a year DOES make a difference.
Your point of pre-existing free services is a whole different matter and has merits of it's own to discuss. I think, as someone pointed out, couchsurfing.org got kinda weird/shady. I was a member years ago and just stopped paying attention (I recall when there were mailing lists/news groups for such). I think money spent on verification makes sense these days.
I used to be very active on Couchsurfing and have hosted dozens of travelers -- to rave reviews, and much more often than I stayed. It doesn't cost me anything to share a couch (or a room, on the rare occasion I had a spare). What I'm objecting to is the website taking $100/yr -- for what? It's unclear -- on principle, when communities like it have existed for far cheaper and far longer. It seems to exploit both the hosts and the guests. I understand some overhead (especially when it comes to trust and safety) but it's not clear to me why that price point is necessary when alternatives can offer the same service for much less.
[0] Yes, okay, fine, the marginal cost is not truly, literally zero: there'll be a few cents for water, a little wear and tear on the equipment, a few squares of toilet paper, a couple squirts of soap, etc. However, most people do not track their budgets that closely anyway, so it would be lost in the noise.
So the fee is really only if you want to be a guest. But then you need to host to earn credits to be a guest.
Any additional benefit you may be able to obtain is contingent upon other people wanting to park on your lawn for an equivalent number of nights first, which implies you're probably not in the can't afford a campsite bracket, is probably more hassle than paying for a campsite, and isn't much use if you're looking to stop for four nights somewhere in the near future...
Other sites charge less for the right to stay 365 days a year at thousands of actual photographed locations without being a California homeowner with parking space, and waive the fee if you host.
I don't think I've met a single millennial in the past year that cooks their own food. I'm worried for them.