wysu
You're getting a bit mixed up here. You can't see well in fog using your own lights -true- but that's using the lights to illuminate your surroundings rather than be the surroundings. You are no longer interested in the runway just the lights of the truck. Don't tell me you haven't hitched a tow from a high speed car that overtakes you on a foggy day.
So how does the truck get around? You have to remember that what you can safely use on an airliner doing 160 mph or along the side of a runway is entirely different to the options you have on a truck on speed-restricted taxiways. Also as you have noticed how much easier it is to drive in a city or along the illuminated sections of roads, similarly your problems fall away as you apprach the terminal complex.
I still think it's statistical - doesn't happen often enough to justify the investment.