> The main one being that airplanes are not equipped with sensors capable of detecting other aircraft nearby. Well, there's ACAS/TCAS (airborne collision avoid

> The main one being that airplanes are not equipped with sensors capable of detecting other aircraft nearby. Well, there's ACAS/TCAS (airborne collision avoidance system / traffic collision avoidance system) and certainly we are moving towards ASAS (airborne separation assurance system), based mainly in transponder / ADS-B radio signals. But as others have commented, there are other reasons (psychological, economic, etc.).