Usually, this is a consequence of flight scheduling. 3 planes come down within 10 minutes of each other, and as fast as people get their luggage, they're in a cab headed for their hotel.
Do most people take cabs? No. Most people have a car in long term parking, a family member picking them up, or are renting a car. If every passenger got in a cab, the airport could tell the cab company they have a certain number of people needing rides next Friday at noon, and the cab company would find a way to have the cabs there to do it. In fact, if it were done that way, all those people would probably be loaded in a bus and taken to a central point downtown, like the train station in Albany, New York, for instance.
How it works is, this time there are 10 cabs at the airport and only 5 people want rides. No more flights for 90 minutes. Do the other 5 cabs wait for 90 minutes? They might, but they are not making any money. You go to work to make money, and you can't afford to spend the majority of your time reading the Times Union newspaper. If there is something else to do, more than half of all drivers will leave the airport empty.
Sometimes, there are 10 people wanting a cab ride and only 5 cabs. What happens then? The cabs will load 2 or 3 of them, until they are all in a cab. That 5th cab might get nobody, and when that happens, that driver is not real happy.
The other main cause is weather, and usually that weather event is snow. If the snow is going full blast, people will beg you for rides everywhere. There won't be enough cabs at the airport. People will walk out into the middle of the street and block your way, to make you stop, so they can demand a ride. It's crazy. It doesn't happen often, but it's crazy. Snow doesn't give you a right to overcharge people. If you're running a meter, the cost will go higher because it will take longer, but snow doesn't justify taking the scenic route or charging a snow surcharge, you just pay the meter rate. The point A to point B rate in a non metered cab, is the normal charge.