Flying into a big-city airport is a complex task under the best of circumstances. Pilots approaching an airfield must make a constant series of decisions.
They're glancing at an approach chart (perhaps displayed on an iPad or other tablet) that's strapped to a leg. They carry on a steady conversation with the airport's tower. At the same time they are keeping an eye on altitude, airspeed and attitude to make sure they are in the right place to keep the aircraft and passengers safe.
While all of this is going on, it's the pilot's job to look outside the cockpit to see any traffic that might get in the way. And of course the pilot is watching the runway to make sure it's clear and that the plane is on course for a safe landing. Something can go wrong during any runway approach and pilots have to be ready to respond instantly to any emergency. Sometimes when something unexpected happens suddenly the results can be serious and even fatal.
This situation is already complex enough. In fact, it's so complex that aircraft collisions occur near major airports from time to time. Sometimes those accidents happen on the ground in a wide variety of scenarios, despite the fact that there may be a control tower watching for problems. As the airspace gets more complex near busy urban airports the difficulty grows along with it.
The first time I landed at a major airport it was at Washington-Dulles International and I was flying an airplane that traveled at a leisurely 70 miles per hour. I had plenty of time to look out for hazards in the air. But these days things happen more quickly; airplanes carry more people, and adding even more craft to the airspace only increases the complexity, which certainly adds to the risk of collisions in the air and on the ground.
This is why the FAA is taking its time approving autonomous aircraft, or unmanned drones, such as those that Amazon is developing as a means of package delivery. Fortunately, for now airline traffic is safe from possible airspace incursions by such aircraft. Delivery drones such as those proposed by Amazon are a lot farther away from becoming a reality than Amazon would have you believe in its Prime Air announcement.
While it's true that the Federal Aviation Administration is working on ways to allow the use of drones, normally called Unmanned Aircraft Systems in government parlance, it's just starting to study the issue. In fact, the FAA's doesn't expect to complete a series of test ranges for civilian drones until sometime in 2017 at the earliest.