Because i was short sighted and pretty bad too as i couldn't read normal sized text such as a newspaper further than a foot way from my face i had to wear glasses, this meant that i could not play any contact sports like football and such as my glasses would get damaged.
This also meant that the first few times i went paintballing with friends was an issue as not only would the crappy rental paintball goggles fog up but so would my glasses (double fog, yay), and almost every year i would have to buy a new pair of glasses as i would break, scratch or bend the frames so would stump up at least another £100 for a new set.
So i said enough is enough and in Jan 2012 got laser eye surgery instead and WOW! no longer needing the glasses when i got into Airsoft my choice of goggles was no longer limited to can i fit my glasses under this or does it fit prescription lenses.
Admittedly Laser eye surgery costs a lot and depending on your eye condition they may or may not be able to do it but i recommend its well worth a look for the long run.