> For health it's pretty much a non-negotiable to go do some strenuous activity every other day, and going to the gym is the path of least resistance to getting that done
You are right, but the reason it's so prevalent is also because it is better for capitalism. Going to the gym isn't just a 1-off activity, it's an entire lifestyle & it doesn't necessarily come cheap. You need a membership, specialized gear, lessons, switch your consumption habits to high protein foods ... etc
> it's an entire lifestyle
You might be overthinking this, although I understand how it can appear that way for someone on the outside looking in. Your local gym probably costs less than your Netflix subscription, if you don't already have a free gym in your apartment building. You don't really need anything else other than looking up some routines on reddit.
Most people at a gym are not the fitness-focused people you're thinking of. Most are just white collar workers who need some exercise. A typical American diet already has an embarrassing surplus of protein, there's very little gear that your average person is going to need (active clothes can just be shorts and t-shirt, maybe swim trunks, and a pair of shoes), and IMO most people don't need a trainer for more than a couple of sessions to show them the ropes.