"I personally think accuracy issues are because security vendors don’t want to show that their expensive software found nothing"

That might be a bit cynical, not sure I'd call openvas expensive.. I think it comes down to the fact that the damage of a single false negative can far outweigh the cost of the false positives. In the end, these scanners are mostly a tool to help document your security efforts, often for compliance reasons, making you think about and document the choices you make from a security perspective.