A side effect of spending many hours inside staring at a screen is that you are typically not exposed as much to outside brightness levels. The level of light that your eyes receive has a drastic impact on the way that your eye develops and not getting enough leads to myopia because the different parts of the eye end up growing at different rates.

Of course, when your eyes have fully developed you want to avoid the bright light to prevent cataracts. It's hard to win in biology.