They don't generally cover normal vision needs. Your regular glasses and/or contacts aren't covered.
Eye injury or cancer, though? Usually covered. I honestly can't recall anyone getting denied for a possible scratch on the eye, either here in Norway or the US (Am from the US). Expenses for eye issues with MS or Diabetes? Usually covered. In these cases, you often go to a specialist MD instead of the normal eye doctor (if you see an MD at all). More is generally covered for eyes than teeth - if teeth were covered like eyes, it would be an improvement. A lot of acute care and infections and stuff would be covered.
As a glasses wearer, not covering glasses feels reasonable to me, although I can't quite put my finger on why.
I guess I see glasses at least partially as a fashion accessory--a necessary one for sure, but then so are shoes, and I don't expect those to be covered by insurance.