Because it's his story.
Canon isn't only in relation to something else, it's also genuine works by said original author.
Saying a fanfiction based on Shakespeare plays are the genuine thing is incorrect, it is not Shakespeare play.
Everything else is fanon.
This isn't a historical document, this is a fictional story.
You're saying the writer of any fanfiction author is an equivalent part of the dragon ball story when it isn't.
For a fictional story only the original author or chosen successor's works are canon.
Saying it's a headcanon is irrational, comparing Dragon Ball to The Bible is also irrational, we are talking about wildly different things.
Any of the video games are paid fanfictions, like all adaptions of a work.
But people like to not see how the owner of a story is the only actual source of said story and anyone else is just fanon writers.
And compare it incorrectly to other things.