I can only imagine that those behind it have no experience of schools beyond having non-SEN children in good state schools where they have been in top sets and achieved good grades and progressed on to university.
I know that having had a SEN child go through state education that I have had my eyes opened in a way that would not have been the case if she had been NT and no dyslexia. Would have been plain sailing and I'd have wondered why anyone wanted to even look at private beyond having shiny facilities and smaller classes.
SEND even in outstanding state schools is a complete mess - and not because the staff don't try. They're under resourced and under staffed and drowning. There are so many children with complex mental health issues that are not being dealt with by NHS and CAHMS that it is falling on school to pick up the pieces. There is nothing left over for those with more educational issues unless you have an EHCP with very specific arrangements.
Just in my own experience - DD is very dyslexic and ADD, but her issues are academic and not behavioural.
In primary they spotted the dyslexia (was rather obvious as she couldn't read by Y2) and did all the testing. None of the things in her Ed Psych report happened - after lots of complaints, her primary said that they had 35% SEN in the class. Most were boys with behavioural issues and they had no resources. So she couldn't be sat at the front of the class - she and the other girls were used as separators between the 'naughty' boys. Too much time was spent controlling bad behaviour - so no time to spend scaffolding, pre-teaching or even just explaining concepts more than once. How do you learn effectively if lessons are spent in corridors because children are throwing chairs? How do you learn when the teacher spends most of the lesson trying to shout over children who are running riot?
Lock down then screwed up her year group - they spent half of Y6, all of Y7 and half of Y8 in various bubbles and lockdowns. So no real way for teachers to see them face to face in an appropriate way before Y9 when they were starting GCSE courses. Even with good online provision there are huge gaps for all children when education is this disrupted let alone those who need extra support above the norm.
I have fought and fought for help for the dyslexia and it has been pretty much said that as long as she passes English and Maths then it's considered job done as there aren't enough resources to support a child capable of top grades to get those when they are passing, as there are so many others with similar difficulties who are not going to get the 4 or 5 they need. Resources are limited and have to be allocated according to need rather than want.
My experience has been that you spend years trying to get information, being fobbed off with vague promises that never actually happen, told not to apply for EHCPs, told that all staff are trained and will be delivering the SEN plan. And that even as an articulate, interested parent who will fight for their child you end up drowning in the system - endless waiting for meetings, for follow ups and then more disappointment. Months and years go by and you wonder why you are still at the same starting point.
I totally understand why many parents remove their children from state in desperation. Especially if the school isn't good to start with, or they have nothing but endless cover teachers, or the behaviour is such that no child could ever learn.
We've been lucky - DD will come out with the grades she needs for her chosen path. She's been in top sets with good behaviour and great teachers where she can learn even if there is nothing extra to help her specific difficulties. The school excels in their provision for her specific interests. She's happy. I can also afford to prop things up with tutors and tech and educating myself on how to help her.
But I have come away realising that our school system and our exam system caters for only a subset of children. If your child is one of those then you have won the education lottery. If your child is not, then it is a very stressful and demoralising experience.