clipped_wings, because it's infrastructure like roads, police, ambulance, fire brigade etc. Has to be there whatever and has to move whatever traffic is there.
Airlines, to be brutal, can go bust and another can take their place. ATC is DIFFERENT and must be there...and the consequences of an error do not bear thinking about!
I repeat, of course it should be run well, but not for profit! We don't expect the police, fire or ambulance to 'make a profit' why on earth should the ATC 'safety service' make a profit????????