Why do so many kids need glasses now?

Wearing "corrective" glasses is anything but. Mainstream optometry exists to keep you a paying customer, not cure you.

"[…] to humans predicts that natural emmetropization will be inhibited in neonates by early intervention with prescription lenses, and that refractive correction of myopia will lead to accelerated progression"[1]

[1] http://onlinelibrary.wiley.com/doi/10.1046/j.1475-1313.1997....

One would think they spend less time looking at far away things.

One would also suspect that opticians might have something to do with this, as literally no optician will sell you glasses which make you see worse, even though that could make vision correct itself and the glasses unnecessary.

Example. Your child has -2.0 eyesight. They are prescribed +2.5 glasses to compensate. They will develop -2.5 eyesight to compensate, making their vision worse in the long run. The cycle begins.