The users are part of the market too.
A lot of people use direct withdrawal from a checking account to pay their mortgage. Because the mortgage lender doesn't have the same incentive as a subscription service to make it hard to cancel.
So you do that for your mortgage, but would you do it for a subscription service you might want to cancel? Heck no. People use credit cards for that. Then if the service won't cancel your subscription you have a last resort the ability to dispute the charge with the credit card company.
Apple makes it even easier but charges even more.
But the choice is really important. Imagine if Apple was taking 30% of your mortgage payment and the bank had to pass that cost on to you. Even the 3% charged by credit card companies would be tens of dollars a month, for an ability to cancel which in that case isn't worth its cost.
So people will choose based on what kind of business it is. If Netflix demands that you use ACH, people will balk. If Apple is providing enough value over the alternatives to justify their fees, people will demand it. And in some cases maybe they are -- customers might demand it for super scummy services they know they'd otherwise have trouble cancelling. So in open competition, the users demand it where it's needed. In the other cases it's not worth the cost.