And that is the key IMHO. Closed-source requires enough commercial incentive by one firm to look at it. Open-source requires enough incentive in the aggregate across many problem, each of whom may have their own reasons... and who cares what they are.

Most bugs I have seen figured out have been a collaborative effort. One person finds one part, which leads to the next person figuring something else out, etc. Much harder to do when it is just your job, and you may not even be paid for this work.