Their personal plan is $4/month.
Maybe one in a few hundred people (let's say 500 for easy math) would want self-hosted version.
Are you willing to pay $2000/month for a note-taking application? I doubt it.
Not to mention the support burden of trying to figure out why something doesn't work on your self-hosted installation.
That economic reality is why on-prem, self-hosted software is almost exclusively a very expensive, enterprise software.
Companies can justify paying a few thousand a year for installation and a salary of an in-house employee to manage and maintain it.
Individuals can't.
If the on-prem version does not exist, I’ll never even try to convince them to switch, since we aren’t going to store anything sensitive on someone elses infrastructure.
So we stick with Confluence. Which everyone hates, but is still the best solution around.
Self-hosting nerds like myself would be able to get by on a no-support 'community edition' for like $10-15/mo.
Personal data notwithstanding we have data we put in our company wiki that is simply not allowed to be hosted on cloud services without lots of red tape and compliance certifications. Offering a on-prem option, at least for us, pushes all that responsibility onto our ops team, and Notion gets paid about the same but without the hosting costs.