Many things are not taught in school, because basic education is short, and a lot of it happens before people's cognitive abilities have fully developed. The average kid probably spends about a year learning about society and culture, and much of it must be devoted to topics that are relevant to daily life in their own society.
Back in Finland some decades ago, there was pretty decent coverage of India in three topics: world religions and the history of religion; European explorers, colonialism, and imperialism; and "modern" history with Gandhi, Nehru, the partition, and the wars. There were also some passing mentions in other topics. Overall, we probably spent more time on Indian history than American history.
> and a lot of it happens before people's cognitive abilities have fully developed.
This is sort of trivially true, in that there is generally literally no point at which a person's cognitive abilities are "fully developed" (cognitive abilities being non-uniform in development, and some aspects of cognitive ability tending to continue developing until very late in life, long after most of the rest have been declining for quite a long time.)
Back in Finland some decades ago, there was pretty decent coverage of India in three topics: world religions and the history of religion; European explorers, colonialism, and imperialism; and "modern" history with Gandhi, Nehru, the partition, and the wars. There were also some passing mentions in other topics. Overall, we probably spent more time on Indian history than American history.