Each field has grown into a silo with "experts" in one field barely knowing about the cutting edge of another field.
Before what was the cutting edge was knowable to anyone who really tried. Because the total body of human knowledge was extremely limited. And that's why we see so many "gentleman scientists". They were people with passive income from property and enough time to tinker where they liked. Robert Brown of Brownian motion wasn’t exactly a genius. Yet you find his name in multiple areas of Science.
Nowaday, the edge of human knowledge is far more spread out, and it is harder to be a polymath. Polymath aren’t people who can "talk about" and "hold intelligence conversations" about different fields of Science/Social Science, they are the people that do multiple areas of academic study/engineering.
EDIT: And knowledge gained in HS is extremely shallow. They don't know even one field enough, let alone multiple fields.
And High Schoolers of earlier years actually knew stuff more in-depth, because stuff weren’t diluted for "inclusive" education. I have seen the Curriculum of British Indian HS, and it was much tougher than today's curriculum. 200 marks paper of two, as opposed to one of 70, more advanced topics each year as opposed to easier topics to bump scores up and make the provincial government look good.
Not exactly relevant to Polymaths, but everyone here knows that one who was educated in 1850-1970 years, knows much more than one educated in later years (as chosen randomly among those who have a degree). I believe education dilution has happened everywhere. Even today HS Physics, Math, Chemistry in India is much harder than US. I have seen curricula and books from both.