I think you might be vastly overestimating the technical competence of a lot of people in non-tech management. In my experience, a lot of people really do believe those nonsense phrases we hear in response to a disclosure - "our system has been audited, so we know it's secure", "we use military-grade password encryption" etc. To the average user, this stuff is basically voodoo.
Imagine that you got a letter from a stranger, telling you that the locks on your upstairs windows didn't work properly. How confident are you that you'd take it as a helpful suggestion, rather than being completely creeped out by this menacing weirdo?
I think that negative responses to disclosure are generally grounded in a mixture of fear, mistrust, misunderstanding and arse-covering. Someone who discovers a vulnerability is seen as inherently untrustworthy, because why else would they be snooping about and trying out the locks? We think of computer systems as inherently insecure until proven otherwise, but they see their systems as fundamentally secure until someone comes along and breaks it. If you're fearful of technology, it's easy to hear "excuse me, I think your system is vulnerable" as "nice system you have here, it'd be a shame if someone broke into it". Denials and cover-ups are often the default corporate response, because being the bearer of bad news can be a career-limiting move in many organisations.