Gyms largely make money from people having memberships but never actually going.
There's only a few types of gyms where most of the members actually use the gym, and although they're still subscription based, they have entirely different business models.
A friend of mine worked in sales for a big national gym. Not understanding their business model, he proposed a program that would generate some excitement among the membership and bring many of them in daily to participate. It didn't get shot down, it just didn't get any interest at all. When I explained it his eyes went wide, like it was a new idea to him. This strategy doesn't seem to be widely shared even with their own sales force.
He left and is now working for a company that actually wants its customers to use its product more.
The gym needs to have a bar making and selling overpriced "health drinks", and also a store selling overpriced merchandise; then getting more people into the gym would actually be profitable.
There are plenty of subscription based gyms that have high utilization and also make it easy to cancel. They’re just usually more expensive (e.g. $200 per month instead of $20 like 24 Hour Fitness).
I'm a regular at one of those pricy gyms, and I think you're spot on. There's high utilization, and the gym actually bugs you if you haven't shown up to class in a while. The high price probably leads to a degree of self-selection among members, and the class-centric nature of the gym (as opposed to just being a floor full of equipment) probably means there's business value to people being there.
I'm a rather happy customer of Planet Fitness and a regular user. It's pretty clear what their business plan is, but the gym itself is fine, obviously with no frills. The most obvious deficiency is that they only have smith machines and no barbells, but that's not much of an issue for me. The strangest aspect is that there are no scales in the locker room; I assume that's a purposeful part of the atmosphere. Still I recommend them to all my friends. If they don't go, I suppose they're still subsidizing my membership.
There's only a few types of gyms where most of the members actually use the gym, and although they're still subscription based, they have entirely different business models.