Here's what I made up in my head:
Corporate managers and lawyers in particular have to constantly monitor for and defend against legal attacks, both legitimate and illegitimate. They have to stay on their toes about tricks and traps built into contracts and business deals and that sort of thing.
When a nerd comes to them to report a true fact about reality that will help them to know, we (the nerds) expect them to be grateful and cooperative.
But in fact they are trying to figure out what the angle is, or if not, what the angle could possibly be. One nerd's helpful security disclosure is a corporate lawyer's extortion attempt: "Nice corporation you got there. Too bad about this critical security vulnerability that may or may not constitute fiduciary negligence, but would definitely harm customer trust in your financial institution. Maybe we can help each other out, friendly like..."
So when someone comes at you like that, what do you do? If you're a hardass corporate lawyer you posse up, lock down, stonewall, shut off any practical ability for the person to have any further interaction with you, use all legal means at your disposal to get them to shut up about the issue now and forever. After all, this person just proved they have the ability and probably the willingness to discover vulnerabilities and extort you with them. Maybe. Why risk it?
That's the story I made up about it. I think it's a combination of incentives in the legal landscape and a huge culture clash.