"but how do we know it doesn't exist to some extent?"
The majority of boarding schools are regularly inspected and boarding and pastoral care are a large part of that inspection. minimum standards are laid down and children and parents are interviewed asked to complete questionnaires, At all the boarding schools i've worked in children are provided with a list of people they can turn if they're unhappy in general children are told all staff will listen but then specific staff are listed e.g. chaplains schools provide a free counselling who children can access confidentially and in confidence. All our housemaster are parents themselves (as are most of the teachers and staff) and work tirelessly for the the children in their care often putting the needs of the of the boarding children over their own children: sending emails to parents at 2 in the morning, accompanying them to hospital, attending pastoral care meetings where unhappy children are discussed and solutions sought, talking to parents both on the phone and face to face at all hours and just being with the children. We all (from cleaners to head masters) attend child protection training. Parents are also significantly more involved many will list pastoral care as one of the most important things when looking for a school, gone are the days when parents drop their DC's off at the beginning of term and pick them up at the end, most parents are in regular contact with their children they know what os going on.
I'm not deluding myself there are children in all boarding schools who are unhappy being there (we have them and even advise parents to seek alternatives and some wont) but the vast majority I believe feel supported and cared for and IME the vast majority who are unhappy have problems outside of school and in fact we are often the safe port in a storm we have children who cant wait to get back to school away from dysfunctional parents becasue they feel secure cared for.
I had a child in a state school with a reputation for high standards of pastoral care misunderstood by his school he became unhappy frankly i felt the school couldn't give a toss about him they were just going through the motions in the end his GP intervened and had to point out in no uncertain terms that they like those who work in boarding schools have a duty of care but unlike us who take this very seriously they were not. This would never happen in any of the boarding schools I've worked in.
There will always be cases of abuse, uncaring staff etc, but this in the 21st century in the exception not the rule.