If outside the aircraft is pretty cold and inside the cabin the air temperature is different, why don't the windows become fogged up on the inside?
The humidity at the altitude airliners fly at is VERY low. However, a tiny bit of moisture does build up - if you look closely you'll see little ice crystals on the window (it's also very cold up there - in the negative degrees F usually). On top of that - airliners have multiple air conditioning "packs" that dehumidify the cabin air so there would be indeed by very little moisture to condense on the inside of the windows. Also, if you look at the windows - they are often two or three layers with quite a bit of space separating the outside of the window from the inside (often multiple inches). Some of this is for pressurization and some of this is for thermal insulation. The VERY cold outdoor air is not in direct contact with the inside cabin pane.
Outer window panel is pneumatically isolated from the outsife, it is a pressure window , the second and third inboard windows are primarily there for thermal and acoustic insulation. Condensation (frost) forms on the inside of the outer pressure window because there is a pneumatic pathway to the cabin and passenger breath contains water vapour which becomes attached to this surface during the flight.