There are a surprising number of one person OS projects, many of them more useable than TempleOS.

What makes TempleOS unique is the fame of its creator, sadly largely as the result of his mental illness. His story also explains how it was able to become so famous while also not attracting contributors.

Other OS projects either stay obscure or, if they become widely known, they attract contributors. SerenityOS is an example of what was a single man effort but is now a reasonably large and thriving community. It has become self-sustaining enough that the founder largely focussed on the web browser these days with the OS connoting to move forward largely via the work of others ( including ports to other architectures ).

I think part of what you are saying though is that most other projects aim for POSIX compliance and that is true. There are some that don’t but, as above, that tends to keep them obscure.