There is some evidence that it’s because children spend less times outdoors. After controlling for that, apparently reading/computer time doesn’t matter.
Incidentally my recent personal experience is that spending some time in sunshine helps my eyes feel better for the rest of the day. I wonder if there is some research on the benefits of sunshine for adult eyes.
Most eyeglass stores seem to carry less variety of frames than they used to. Most of them are the small rectangular style that the opticians invariably promote since they can use a thinner lense.