It doesn't need to be queried "why" there is a rise though because you can't ask "why" before you have asked whether there IS even a rise, and this is actually already known. Whatever people's personal experience with SEN children over time or anything else.
I know I keep banging on about the Warnock report but you can quite freely find it online and it was a highly respected study at the time and was the basis for the current Education Act/Children and Families Act.
It's also old enough (late 1970s) that it fits into this idea people had of there "not being so many" children in need of support in the past.
Interestingly they themselves had this to say about causes:
First, we have not felt it part of our business to go deeply into the factors which may lead to educational handicap. We are fully aware that many children with educational difficulties may suffer from familial or wider social deficiencies. While for most children their family life enhances their development, others show educational difficulties because they do not obtain from their families or their social circumstances the quality of stimulation or the sense of stability which is necessary for proper educational progress. But regardless of the cause of such children’s problems, familial or social, unless part of their educational provision is designed to compensate for the deprivation they have suffered, they will be unable to benefit from education in the ordinary sense. One cannot always keep these different strands apart.
They actually make it clear several times in the report that pupils with greater educational need won't necessarily have any kind of medical or diagnosable condition, sometimes the need is just due to aspects of the child's life which are outside their control e.g. poverty, a chaotic home life, English as a second language etc. And that they are including all of these things under this umbrella. So the idea about "people with SENs" (meaning presumably diagnosable conditions) is not actually what it referred to originally or in this context.
In terms of the extent there is a lot of discussion on page 38 if you want to look for yourself, but this is a good summary:
The evidence of the Isle of Wight survey, the inner London survey, the study of the infants’ school and the National Child Development Study broadly suggests, therefore, that at any one time about one child in six is likely to require some form of special educational provision. This is not of course an exact figure. It will vary from area to area according to local circumstances and will be influenced particularly by housing and other social factors and the character of individual schools, including their location, buildings, organisation and staffing, the effectiveness of their teachers and their approach to discipline. All these may affect the incidence of special educational need, especially in the realm of behaviour. Nevertheless the figure of one in six represents what we believe to be a reasonable judgement on the evidence and, indeed, is in line with the estimates of the number of children who might be expected to require special education which were given in 1946 and to which we referred in the last chapter.
Some other figures from that chapter are that in 1976/77 in England and Wales, 1.8% of pupils were in special schools, and around 4.7% spent at least some time in special classrooms set up in about 40% of mainstream schools for children who had "difficulties in learning, or problems of an emotional or behavioural nature, or both"
...we estimate that up to one child in five is likely to require special educational provision at some point during his school career. This means that a teacher of a mixed ability class of 30 children even in an ordinary school should be aware that possibly as many as six of them may require some form of special educational provision at some time during their school life and about four or five of them may require special educational provision at any given time. Again, we should stress that these figures will vary from class to class, school to school and area to area. The figures will however be an essential guide for planning purposes and we recommend that the planning of services for children and young people should be based on the assumption that about one in six children at any time and up to one in five children at some time during their school career will require some form of special educational provision.
(In that last quote, the bolding was italics in the report.)
So - 1977 figures, 6.5% of pupils spending at least some time in a special school/special classroom, 1/6 - 1/5 (16-20%) depending on how you count, needing SEN support. Backed up by 1946 estimates which were similar.
2024 figures, 4.3% with an EHCP, 17.3% needing support.
Where is this rise? I am not seeing any rise. The numbers are totally static, maybe a bit lower than they should be. When was the time where children didn't need any extra support in schools? It was recognised in the 1970s. It was recognised apparently in the 1940s.
The only possible way that there could have been a rise is that at some point during the last 50 years, we have stopped supporting children at school and now we are catching up to where we used to be.
As for why posters are noticing an increase in pupils with certain types of learning disability over others, I don't know how to explain that if indeed the overall numbers have stayed the same. I wonder if this is more due to changes in provision with special schools closing and most schools no longer having special education classrooms.