Comments by "James LaBarre" (@SenileOtaku) on "New York's Disastrous 1960 mid-air collision Explained" video.

Probably because if planes could fly in any random location it would be all that much harder to keep track of them, and probably result in more collisions. The pilots themselves would have to constantly look for planes operating in unexpected locations. With specific flight corridors they'll have more of an expectation of what normal operations will be.
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