Until the 1960s or so, most cars had "generators," which produced DC. They had heavy permanent magnets, and brushes and commutators that wore out. Around that decade, cars switched to "alternators," which generate AC using field coils, with no permanent magnets or brushes. They're smaller, lighter, more reliable, and generally have higher output. In a car, the AC output is rectified to DC by diodes contained in the alternator itself. In many aircraft, the power is distributed as AC for the reasons others have pointed out.