I know what fanjo is saying, and agree that, to an extent, some of these children will have undx'd SN. especially since the dx process is becoming more and more of a 'wait and see' process - delaying the dx (and thus any help the parents or child might need) beyond school starting age.
I know a child (in fact I know several) who seemingly did not recognise a single nursery rhyme; or know how to open a book. who can only drink from a toddler cup/sports bottle. who was in nappies as they started school. and so on.
of the children I know to whom this applies, all now (at the age of 8+) have a dx of some sort. my own (who is among this number) has severe ASD. others have dyspraxia, or AS, or GDD, or a whole host of other things.
I am not saying that deprivation does not happen. Sadly I know it does. Same for neglect. but it is not always the case.
I would think a more interesting question is: why, amongst the many middle class homes that I know, are these issues so prevalent now, when they were not a generation ago? and I am talking about issues now, not neglect or deprivation. children from homes where they are loved and looked after. who have toys, but cannot play with them, despite their parents' best efforts. who have books, but cannot open them, let alone read them. who are sung and read to regularly (to the point of exhaustion on the parents' part) but cannot speak in full sentences, or recognise letters/sounds/numbers/whatever. because there are an awful lot more of these children than there ever used to be.