At least one of school names that was mentioned in news reports is a PRU, you don’t get sent to one of those for no reason.
It is quite common (having worked in one) for the children in PRUs to have parents who either underestimate their risk level (“not my Timmy”) or struggle with their children to the point they disengage. We had parents wouldn’t answer the phone, or tell us to deal with it because they’d had enough, etc etc.
We had kids who would threaten to come to your house and burn it down, and if they knew where you lived I have no doubt they would have.
All of them though, are struggling families, and those children don’t present any less risk in the home than they do outside of it.
I just don’t think many can fully comprehend the absolute terror of your own child presenting you and others with that level of threat, and don’t really know what they’d do or how they’d cope in a similar situation.