Not being able to steal a jet is just bad luck (got one yesterday), sometimes you are just f*ed. This can for example be that when you enter the hangar two tanks are already close by and blow you up before you have entered cockpit properly. Or a barrack park infront of the opening causing you to explode when crashing in to it. Both these things happened yesterday, but if you go back and try again after you die it works. I could steal one and go away, was waiting for missiles or other jet but nothing ever came, I just ended up going away from the base in a straight line with no resistance.
So if you can't get one, pure coincidence. Try again.