When I started learning to fly it seemed impossibly difficult. Flying seems easier now, although still challenging. It's roughly the same kind of skill as driving a car and no one thinks back on learning to drive as particularly difficult. But it only takes 6-10 hours of driving time to get a car license, compared to 40-70 hours of flying time for a pilot's license. Why is flying so much harder?
Control complexity. Cars only steer in one dimension; planes steer in two. Even a level turn is hard in a plane, you have to coordinate two controls, except sometimes you deliberately uncoordinate them. Managing engine power is harder in a plane: two or three controls in a piston, not just a single pedal. And then there's auxiliary controls you have to use occasionally: flaps, carburetor heat, fuel tank selector, etc. Even starting a plane requires carefully using four controls in the proper relationship.
Being in the air. Planes at altitude have a lot of potential energy. A primary challenge of landing a plane is learning how to bleed that energy off to make a soft landing. In a car you just brake. And if something goes wrong in your car you can pull over to the shoulder and stop; that doesn't work so well in a plane. So there's hours of learning how to get a plane safely down in various kinds of emergencies and equipment failures. Also learning to ensure you can land: running out of gas or flying in to clouds are dangerous, so there's a lot of training to avoid those situations.
Instruments. Operating a modern car is pretty easy, you don't think about the engine unless an idiot light comes on. Flying requires a lot more vigilance. You're constantly watching your heading, airspeed, altitude, attitude, and engine power to make sure you're flying roughly right. And there's a bunch of extra instruments you need to scan regularly; oil pressure, outside temperature, voltages, etc. It only gets more complicated if you're flying when you can't see, that's a whole 'nother 55+ hours of training.
Communication. Most general aviation flights are constantly on the radio, talking to ATC to get permission to takeoff and land, to get separation from other aircraft, or simply to be followed for safety. We know talking on a cell phone while driving is remarkably distracting. It's not quite as chatty in a plane, but it's necessary communication and it has its own special language. Learning to work the radio was remarkably difficult for me and it's still a constant distraction.
Navigation. In a car you just follow the road and signs, maybe look at a map or a GPS to make sure you make a turn. In a plane you're largely on your own in three dimensions. If it's good weather it's not too hard to avoid mountains, but some of the obstacles are invisible. And while moving map GPS has revolutionized GA navigation, I still have to learn to fly without it: dead reckoning, pilotage (aka "looking down"), flying between radio beacons. And even if you know which way to fly, you still have to calculate for invisible winds.
The good news about pilot training is it seems impossibly difficult at first but gets easier with training. In a followup blog post I'll talk about some of the ways pilots manage complexity. But no matter how skilled the pilot or how many good tools they have, flying always remains a complex activity requiring attention, thoroughness, and constant practice.