I agree that flying is pretty safe compared to other forms of transport and there's no need for people to wish you a safe flight. When someone wishes me a safe flight, I just say tkank you without really paying attention to their choice of words. I'm just happy they care about me. Personally I wish my friends and family a great flight, but I don't think they would mind it if I said "have a safe flight". I actually never really noticed that issue until you mentioned it. I still think it's comforting for some people to hear that if they're scared of flying.