Because there is no actual 'face' slot, just a 'head' slot, so you can only have one or the other. There are some hats that also include glasses with them (which can be hidden or made visible at will by using the /visor command), like the SCH AF1 mortarboard, but unfortunately they're not going to make a seperate slot for glasses/facegear, for two reasons.

Because that would require a complete rebuilding of the equipment system (specifically, the equipment screen) which would demand development time and resources that could be better spent elsewhere, and secondly, there would be a problem of having 'face gear' potentially clipping badly with headgear, which would restrict gear design (this is why belts were changed from 1.0 to ARR, as belts having an actual appearence meant body gear design was restricted to prevent clipping, which SE decided to sacrifice in order to create more interesting armour designs. It's the price that has to be paid unfortunately.

So alas glasses will continue to be considered 'headgear' I'm sorry to say.