I suspect there's many reasons for the field/department cultures.
One of them, which was surprising to me (which I first heard from a friend in a hard STEM field), was what happens when student A's thesis result is found to be wrong due to flawed experiment... but only after student B is well into their own dissertation building upon A's result. Reportedly, everyone involved (A, B, their PI, the department, the university) has incentive to keep quiet about student A's bad result. B has an academic career to move forward, within funding and timeframes, and everyone else cares about reputation and money. And there is only downside for bystanders to complain, especially if it's other students especially vulnerable to retaliation/disfavor.
Another one I've seen, which is less surprising, is when there seems to be a culture of alliance or truce among faculty. So, if someone is misbehaving, or makes a mistake, it's understood that no one is going to call them out or interfere, and no one wants to even know about it more than they have to. In general, no selfish benefit can come from that, but a whole lot of negative feedback can. Mind your own business, glass houses, etc.
One of them, which was surprising to me (which I first heard from a friend in a hard STEM field), was what happens when student A's thesis result is found to be wrong due to flawed experiment... but only after student B is well into their own dissertation building upon A's result. Reportedly, everyone involved (A, B, their PI, the department, the university) has incentive to keep quiet about student A's bad result. B has an academic career to move forward, within funding and timeframes, and everyone else cares about reputation and money. And there is only downside for bystanders to complain, especially if it's other students especially vulnerable to retaliation/disfavor.
Another one I've seen, which is less surprising, is when there seems to be a culture of alliance or truce among faculty. So, if someone is misbehaving, or makes a mistake, it's understood that no one is going to call them out or interfere, and no one wants to even know about it more than they have to. In general, no selfish benefit can come from that, but a whole lot of negative feedback can. Mind your own business, glass houses, etc.