The theory goes something like this: the sky is SO big, there is no way that two planes could collide. Sadly, that theory's not quite true. Most of these accidents indeed occur closer to the ground, and, around congested non-controlled airports. However, planes come together tens of thousands of feet above ground in cruise flight. When a United Airlines and TWA flight bent metal above Arizona in 1956, the FAA was created to provide separation of planes. We still come close and planes still collide. I can't count the number of times I pass over, or under, another plane in cruise and think, "If I were a thousand feet lower, or higher, they'd be reading about the crash in the news." The good news is that we have very sophisticated traffic collision avoidance systems (TCAS) that alert us when planes are intruding our space, and, tell us how to maneuver when coming together is imminent (when planes approach at over 1,000 MPH, the computer can calculate the escape route better than us). Oddly, we're also allowed to fly closer together than ever before (1,000 foot separation, where it used to be 2,000 feet separation). This is due to Reduced Vertical Separation Minimums (RVSM), which has many requirements, including 2 independent-source altimeters, altitude alerting systems, pilot training, aircraft certification and more. We still need to keep our eyeballs outside, but it's the technology that often saves us.