Exposure to very bright light (even standing under a shade tree on a cloudy day is far brighter than normal indoor lighting) specifically during key years had been nailed down with high confidence as the primary cause of most near-sightedness, last time I looked into this.

Not sure if they'd figured out the mechanism, but speculation was that bright light is involved in signaling the eye to stop lengthening at the right time.

IOW the cause is, basically, school. In Winter months, kids may hardly see the Sun at all, with very short days and recess often indoors (too cold, too snowy, too wet) especially as schools have seemed to grow more wary of taking kids out in poor weather, over the last few decades. Plus anything else that keeps kids indoors, sure (video games, social media[0]) but school seems to be a lot of it.

[0] And actually, the social media factor likely kicks in well after it matters. Ditto video games unless your kid becomes a gaming addict in the 3-7 age range and you just let that happen.