Why do you make healthy, nutritious meals for your DC and restrict sugary snacks and junk food?
Why do you read them bedtime stories, and talk to them about the world?
Why do you comfort them when they're frightened, and help them understand and deal with their emotions, talking through problems they have with their friend?
Why do you take them swimming, and out to the park, and encourage them to try out different activities which help them strengthen and learn?
It's all the same - and it's the most natural, healthy thing in the world. Parents want to help their children to grow and develop, and to become the very best version of themselves they can be - because being the best version of themselves helps the child every single day of their life in every way.
Why on earth do you see schooling as any different to any of those things?
Sure, the state provides a tax-payer funded education for any child - just as it provides libraries, playgrounds, free council swimming sessions at council pools, free breakfast clubs at school, and CAMHS support for mental distress. Do you judge people for buying their DC books, signing them up for a bushcraft holiday club or for swimming lessons, for making them a healthy breakfast before school or talking through emotions from an early age?
Do you think all those things should be illegal - and children only given any kind of development and support if the state provides it?
Do you think it's wrong of you to have maximised your child's physical health by feeding them well through their childhood, and grown their character and mental health through strong parenting and healthy childhood experiences? Would it be better for the child with a worse childhood if you hadn't supported your child as you have?
Should your child be deliberately held back in university and career from making use of the health and personal and emotional strengths you helped them develop?
And before you say 'but school's different. It's OK for everything else to be completely unequal in childhood, but not school', just think clearly for the tiniest second about whether that's true. What do you think is more important for your child's future happiness and success? Your parenting (including all the things I gave above) or their experiences at school?
So why do you think everyone makes such a huge deal of private schools, to the point of mocking private school children and actively trying to destroy their education?
(clue: it's sure as hell not for pro-social or altruistic reasons)