Why don't eyes fog up?
You can fog up the surface of the cornea.
But in order to condense water on a surface the surface temperature has to at least reach the dew point temperature for a particular barometric pressure and relative humidity.
Typically if you are inside a building for example with air conditioning and walk out into warmer, humid air, glasses can fog since they can be of lower temperature, un-warmed by the body.
But since your body generally keeps the surface temperature of the cornea warm (I would guess between 30 to 35 degC), you would have to have very warm temperatures and highly humid conditions for that to happen. But the physics say it can happen. Good thing we have built in windshield wipers (eyelids).