I think it's actually because people don't like spending money, even very small amounts, for things they have decided fall in the 'free' category. As previous poster said, it's a monkey problem, not a tech problem.
That is also why the Google and apple appstores can charge such large fees. Sure, buying on websites and sideloading are possible but much higher friction -> many customers simply won't bother.
That happens less because they don't carry cash anymore.
It's the same thing here.
If it was easier to do, more people would do it.
Look at Onlyfans. People pay good money for what you can get for free.