In Mark Manson's new book[0] he talks about how humans can excel at very few things, often barely one, and that "being a good person" for lack of a better phrase counts as one. It seems obvious in hindsight, but struck me because I hadn't thought of it that way before.
(The book is incredible, by the way. I've already read it twice, just like Derek Sivers. Recommend it highly.)
I haven't read the book, but this doesn't seem to fit with what's known about general intelligence, nor the examples of polymaths from the renaissance period.
I suspect the causes are societal, from the constraints placed on our time and freedom to explore and become well rounded.
(The book is incredible, by the way. I've already read it twice, just like Derek Sivers. Recommend it highly.)
[0]: https://markmanson.net/books/subtle-art