Although severe prematurity is one contributory factor to disability, it's not the only one. Severe early illness of any sort, and genetic disorders, are also less likely to lead to early death than was the case 30 or 40 years ago. Which means that quite a lot of children who would have died in infancy now survive long-term, in some cases with disability.
But also there's something else: until (I think) the 70s, and to some extent beyond, children with severe intellectual disabilities were deemed 'ineducable' and whatever provision was made for them, did not come under the Department of Education or local education authorities, but usually under what was then called the Department of Health and Social Security. They stayed at home and attended 'training centres', or very often were placed early in institutions which made no claim to be residential schools but were referred to as 'subnormality hospitals'. Thus, often such children were not known to the general public at all, and at any rate not in terms of their need for educational provision.
In any case, there is a very upsetting tendency in some of the media for people with mild/ moderate disabilities to be viewed as scroungers and people with severe disabilities to be viewed as a poor economic investment (shades of the Nazis, with their 'useless eaters'). Spending on educational provision for them is viewed negatively. And if a parent, usually a mother, can't or won't send her severely disabled child to a school, and instead stays at home to care for them, then she gets viewed as a scrounger for not going out to work. You can't win.
A humane society looks after those who need it.