We’re plural. Our lived experience runs contrary to your opinion.

We aren’t born with our personalities already intact, they develop and unify around the age of 5-6, and DID is a response to trauma from before that age.

That’s the current prominent theory, but testing it empirically would be disastrously unethical.

Here’s the relevant section of the DSM:

https://archive.org/details/diagnostic-and-statistical-manual-of-mental-disorders-dsm-5-pdfdrive.com/page/291/mode/2up

All dissociative disorders (quite possibly all disorders, we haven’t read the whole DSM) require the presence of clinically significant distress or impairment in order to be considered a disorder.

There is no specific entry for nondisordered plurality for the same reason that there is no entry for individuals who do not experience mental disorder: The DSM catalogues mental disorders, not the entirety of the human experience. It’s beyond the scope of the document.

Here’s a good introductory primer to plurality:

https://morethanone.info/

They use tar.