You are spectacularly missing the point. Parents send their children to private school for a variety of reasons.
I would say
One or both parents went to private school so they want the same for their child
There are families that send their children to a particular private school because generations of their family have all gone to that private school.
Not wanting their children to go to the local comp because they see how these children behave outside of school and don’t want that for their child
Their child has an SEN and having received very little help through primary school the private alternative seems a much better fit.
Their child is ND and would be a target for bullies
Their child is shy and overwhelmed at the size of the classes
They might choose private because the school terms are much shorter and they get off peak deals on holidays
Or the local private school is the closest school to their house by several miles
Or because their child is bright and NT but would struggle with the noise and interruptions during lessons
Thr whole point is that those children sent to private school, a lot are just looking to survive education and come out of it unscathed
3 A levels at a decent grade doesn’t even cross people’s minds
You seem to think that is the only reason to send a child up private school