That may play some part in it at least for some people. But there was a time before universal formal schooling. In fact, the average number of years of formal schooling was drastically less in my grandparents' generation than in mine. My grandparents (the three whose lives temporally overlapped mine and whom I knew) did not strike me as any more intellectually curious than their children (seven altogether) or my sister or all my cousins. People born around the same time as them in general did/do not strike me as particularly open minded and intellectually curious.

What I suspect is that open-mindedness and curiosity vary in the population mostly due to genetic factors. Open-mindedness is tied to temperamental factors controlling the balance between novelty-seeking and risk-avoidance.

My daughter is in third grade. There is a lot of creative activity in school these days. There is creative writing in Finnish nearly on a daily basis. There is so much of it that she has begun to dislike those exercises. School is in no way stifling her creativity. On the contrary, it is constantly trying to squeeze it out of her.