They exist! https://screenrant.com/best-documentary-movies-about-normal-people/ These don't all fit, but the first one on the list is about the documentarian's parents.

I think the documentaries that we see are going to be the ones that conceptually interest enough people that they can get, like, funding for it. Or it's just someone's movie they made about grandpa that only people in the family see.

Oh, this question reminded me of NPR's story corp. https://www.npr.org/series/4516989/storycorps
https://storycorps.org/about/