One explanation would be that Substack provides discoverability of authors who readers would otherwise not notice. But I’m skeptical of this explanation, because my personal experience of discovering posts on Substack has rarely, if ever, been driven by Substack discoverability features. Most Substack sites I’ve found via link aggregators or Twitter.
So if Substack isn’t providing discoverability, then their main value is providing publishing and monetization tools. That might make for a nice product, but it’s not a defensible one. If writers could substitute Substack with a self-hosted product providing the same monetization and publishing tools, without affecting the discoverability of their posts, then they would have no reason to stay on Substack if it introduces features that harm their main value of monetization and publishing tools.