I think the high fees on substack, like on patreon have another reason. It creates a weird parasocial relationship between the creator and the reader that gives the reader the feeling that they have contributed to a cause, or has some personal relationship with the author they support, and so on.
In that way it seems to me more similar to twitch streamers who often extract huge sums of money from their donors, when objectively the viewer gets more content out of a 10$ streaming subscription.