While I don’t work specifically in dev, I have worked in security for way too long and totally understand what you mean. (I am too old and too salty to work in this field anymore, to be honest. Technology has changed, vulnerabilities are still wild and different but security as a whole? It has been mostly the same.)
I personally wouldn’t expect a jr. developer to clearly articulate a possible security problem to their manager. It’s hard, to be honest. That could be part of the issue as well.
The laziness I am referring to can happen at any level and in hundreds of different situations. To your point, some people “simply don’t know what they don’t know” and may be just an honest mistake.