Confirmation bias. If you’ve already made up your mind that there was no foul play, in the future there’s no reason to look for evidence of foul play.
This approach was evident very early on in this case, when LE towed the car almost immediately upon finding it.
How hard is it to submit DNA?
You need cause, just like how the effort to get a geofence warrant sputtered. Even the use of gemology databases to solve cold cases, despite the upside, has been contentious. Montana and Maryland just passed laws restricting its use. (NYT) Just earlier this year in NY, an appellate court barred the use of ''familial DNA' until legislators pass a law explicitly authorizing its use. (Gothamist)