If you're doing root cause analysis of a system failure, you're not looking to blame an individual. If you find something malicious then, sure, deal with it. But assume good faith in all parties from the outset and you're more likely to succeed in fixing the problem.
True, the postmortem should not be focused on blaming any one person. But that doesn't mean you shouldn't ignore the presence of such crimes or prosecute them!
The key answer is in what you wrote: "whose job it was". People do jobs mostly to get paychecks, so like water flowing downhill, will avoid investing effort into anything they can avoid. The students OTOH, had a strong non-monetary motivation driving their efforts, so they obviously dug deeper and more thoroughly.
- Adults are not super humans and can make mistakes
- and 17 year olds are as capable as adults.
In engineering people frequently ask - why nobody thought about it - well because nobody did.