Proprietary code is subject to a market value, companies can actually invest in a codebase to improve it and add features, whereas open source is subject to common interests or volunteering. I agree that open source can increase quality thanks to the "community", but unless there is a community, nothing gets done. "there is no such thing as a free meal".

If you don't have the source code, it's actually a little harder to find a vulnerability since all you have is a big blob of binary assembly. Hackers can still find vulnerabilities with enough time on their hand, but it's still much discouraging.