I've been teaching for a long time now. There are many issues.
The demands of the curriculum, school timetabling, school day are unmanageable for a lot of children SEND or not. Schools can't/won't do much about that.
Both parents working full time, the impact of that on wider family support (eg grandparents, aunts, uncles etc also working full time and reduced capacity for support). Schools can't do much about that.
Changes in parenting approaches and the the effects of that. Schools can't do anything about that.
Use of screen time in homes for whatever reason and the impact of that. Schools can't do anything about that.
Some children with 'low level' autism and ADHD who would have coped in school 30+ years ago (like me, for example) and flown under the radar now don't because of increased demands.
School funding cuts meaning fewer resources (human and physical) which leads to a teacher in a class of 30 with 1/5 significant SEND needs (eg complex behavioural needs; significant cognition amd learning needs; reduced capacity to meet the needs). And variable responses to SEND needs in the classroom based on individual teachers knowledge, experience and expertise.
Higher expectations/demands on teachers and children overall - some of which are unrealistic.
Punitive responses to school absence.
Ineffective mainstream inclusion policies and strategies and children who would genuinely benefit from specialised provision being denied access to it due to funding spaces.
The negatives impact of the Local Offer approach to funding SEND.
The impact of these can cause very unsettled and chaotic homelives for children and create issues that mimic SEND. Especially with regards to ND traits. But no one has the means or capacity to address these, certainly not individual schools.
They do, in effect create 'special (or additional) educational needs' in some children that aren't related to a diagnosable condition and who wouldn't be diagnosed with anything but do present with behaviours that make it difficult for teachers to teach and for themselves and others to learn. They just have a different root cause. Eg trauma responses can manifest in ways that look like adhd and autism to a non specialist observer. (The Coventry Grid exists for this reason).
Not all strategies that work with ND children work because the root cause is different. But schools defer to 'possible SEND' because it's easier.
Eg I taught a child a couple of years ago who we knew was living in a misogynistic, hyper masculine DV environment and his behaviours reflected that. This was rejected as the reason for his behaviour, and I was asked if I thought he had ADHD and given a pair of ear defenders for him to use. Because schools have ear defenders but they don't generally have strategies for dealing with traumatised children.