My son is 6 and still in nappies, and is autistic tbh.
The amount of times we have tried toilet training and brought in other agencies like the incontinence team, HV, Eric, GP, communications team etc can't help him because he's not developmentally ready.
I know the OP excluded children with additional needs but a. A lot of additional needs aren't diagnosed at this age and b. It isn't lazy to be having to change your child's nappies and pants every time they need one, it's more effort, so like you say... it would be easier and less energy consuming for kids to just learn to use the bathroom.
Something I noticed specifically before my sons diagnosis and before all of these interventions though was that toilet training advice seemed to change overnight.
In his infancy, we were constantly being told that we had to wait for signs of readiness. That trying too soon would cause bladder issues, urge incontinence, anxiety, trauma etc, and so we would be best waiting for signs of readiness. This was advice we received from the NHS when I was trying to toilet train from 18 months.
Then when he didn't show signs of readiness by 2 and a half when he was in nursery and children were getting ahead of him and I re-approached this with the NHS, the health visitor basically scolded me saying "why are you waiting for signs of readiness? That's a complete myth and children don't need to be displaying signs of readiness to learn to use the toilet!"
If I received this conflicting advice then I am also sure that hundreds or thousands of other parents also received the same terrible and conflicting advice too.
When we started trying again, the nursery told us they did not want my son in pants at nursery, they wanted pull ups at nursery and pants at home because it isn't their job to toilet train, and until children can consistently use the toilet at home they want to minimise accidents and staffing ratio issues by having children in pull ups to make accidents easier to sort. - well that inconsistency isn't going to help any children, but again we are in the middle of a childcare crisis too where there just aren't the resources to help children access the toilet. This is a nationwide issue, and again I bet I'm not the only parent to have faced this.
We obviously did try the 2 weeks off, no pants on in the house, going to a routine, praising lots, not making a big deal out of accidents, but then the transition back to nursery just caused more accidents and regression.
I think putting this entirely down to lazy parenting or SEN is not factual nor helpful. I think it's a complete lack of resource, no sure start centres, not being able to make informed choices because the information has changed so often, and a lack of consistency due to environmental factors.
I'm not saying there aren't lazy parents out there, and I'm not making a blanket statement, but I am just pointing out that a lot of information has changed, and the support you receive when you reach out is very minimal.