Because individual users don't give a lot of revenue to any SaaS company. That's why Notion just made their personal tier free.
It's a top of the funnel to convert people at workspaces into using the tool because they're familiar with it (not talking about Obsidian specifically, but in general).
Why not have a somewhat open core model? The user is free to use the open source app in their local machine, but then there is a license that says you definitely can't host this on your own and charge for it.
Meanwhile for the commercial customers they can offer a managed solution.
It's a top of the funnel to convert people at workspaces into using the tool because they're familiar with it (not talking about Obsidian specifically, but in general).