Myopia, at least for children, could be the result of lack of exposure to sufficient light during the growing phase (https://www.youtube.com/watch?v=DjdkbcOx05A).In the research, chicks with exposure to 10000+ lux had prevented myopia, while the other chicks which didn't get exposed to enough light developed myopia. The overall recommendation from the study was for children to get at least 3 hours of outdoor activity, even during school. The research showed no correlation of myopia and close work (reading, computer screen) or physical activity (exercise, play). Rather, myopia rate was lower among children who had spent more time outdoor, and also had more exposure to brightness of light.

I can testify that it seems true, at least for me, because I'm the only one in the family that has severe myopia, and the only difference between me and my family member is that I didn't go outside that much and stayed mostly indoor. Later in life, I had LASIK, but wearing glasses was awful.