Agencies (even of one) also get economies of scale that gyms don't. You have templates, pre-built components, domain specific knowledge of how to handle common problems, etc.

You also have subcontractors that you can farm stuff out to or help when volume surges. I think the “of one” is likely not always true.

From what I've seen, it's pretty universal for contracts to individual freelancers like this to have an explicit "no subs" clause or at least require prior approval of specific subcontractors. I guess you could lie, or try to negotiate your way out of it in some cases... but I wouldn't count on that at scale.

Gyms have machines, and personal trainers have basic starter programs.

But who pays $2500/month for nothing?

Some, but not many will pay that for nothing. But many pay $2500/month for peace of mind.

I've had contracts like that renew for years with little work most months because a client just wanted to know I was incentivised to be available when they suddenly needed me.