There has been a global issue with school attendance since covid- including in countries that have much better social welfare systems and SEN provision. So we can surmise from that that the breakdown in social contract between schools and families is one of the reasons. I actually spent the early part of my career working in education and crises and one of the maxims always was- keep the schools open. In war, natural disaster, health emergency- keep them open. Because they're such a core part of the social fabric and precisely because as some posters have pointed out, it's so difficult to claw back once you've done it.
I also grew up with a parent who worked in school attendance (they used to go round to kids houses and get them out of bed!) so have seen this over the longer term. There have always been a hard-core of families who persistently miss school. This is often due to poverty and chaotic lives- given the uptick in poverty and the continued cuts to the social welfare net, this will be more families than before. EBSA has also always existed. It's not a new phenomenon. The first line of support for this was EWOs who have been cut across the board. They could support with linking to other services, negotiate between parents and schools or for e.g. social work if needed. That's all gone and been passed onto schools to manage themselves, which removes to advocate role from outside of the school.
I was at primary in the 80s and we had standardized testing (remember Richmond tests?). But they were low stakes. The teaching my year 2 child has is boring (mega chain MAT). But she still likes going to school. We had some bloody boring teaching in the 80s too- SPMG books were coma inducing. The standards of teaching were often very poor. Behaviour at my secondary was really terrible. There were massive safeguarding issues. None of these things are new and lots are much better (I had a teacher in year 4 who would spend an hour every afternoon playing us Christian songs on his acoustic guitar- imagine the uproar now!).
We also need a schools inspectorate. Without out one you have wildly differing standards across schools and poorer kids lose out. But the way Ofsted and school interact is causing some weird behaviours in schools, and that needs to change. Some posters upthread have said how their schools are lovely vibrant places, so it's possible to do, but you have to be a brave school to go out on a limb and do it and probably not part of a big MAT.
I also have a child with ASD who, shock horror likes school. She likes the routine, the dullness, the discipline. This could change later of course. She is really very obviously autistic, but I am not sure things would have been better- in fact they would have been worse. She would have been shouted at to make eye contact, there would have been no reasonable adjustments or extra support. She might, I guess, have been left to do not much work, which probably has changed.
So it's also not universal that all kids with SEN will be refusing school or hate it. We know lots of other children with ASD who are attending and well. But when you combine a lot of these factors together, with Covid as the fuse, that's what happens. Yes, individual children will have different reasons for non attendance- but at a system level, that's why.