I think I've said elsewhere that in my view the G&T programme was badly implemented, but it was put in place because there were concerns that many schools weren't providing appropriate differentiated education for the brighter children as they were focussing too much on their SATs results (or whatever they're called these days). So, the intention was good, but the reliance on statistics (the 10%) was, in my view, a mistake, because there's too much variability between children in primary schools, and also too much variability in how each child develops over time. But the concept was introduced, and obviously some parents will run with that.
But there is another issue, as mentioned by other posters, and that's to do with the history of childhood. It's not been that much time since the days of babies being fed on a 4-hourly schedule (and pretty much ignored in between), and expectations that toddlers were expected to quietly play on their own while their mother/carer got on with the housework, and young children were expected to be out of the house during the day and not bother the adults. (Yes, I'm old, that was what my childhood was like.)
Changes happened around the 1960s or so, along with a whole load of other changes. By the time my children were born in the late 1970s, DH & I did have different views on raising children than the previous generation, in terms of reacting to a child's needs rather than seeing them as an object - demand feeding was regarded as a "fad" back then.
It's taken a long time for awareness of how babies'/young children's brains work and develop, and understanding how amazing that is. It doesn't surprise me at all that some parents are astounded by how brilliant their young children are - but I bet that's quite often because nobody told them that that's what young humans are like.