In short: Because they do not have to and it is easier just not to answer.
The truth however is they are not evil or do not care about you. Airlines and most general companies get Job seeker emails and phone calls everyday, even without advertising or looking for Job placements. When engaging in a conversation a company:
A – has to either pay someone to directly answer hundreds of emails or pay someone to supervise a system that automatically answers hundreds of emails everyday.
B – risk upsetting you or getting quoted out of context by you.
By not answering job requests a company leaves things up to your imagination, which can be frustrating to you, but is safer, easier and cheaper for the company or airline.
Do not be discouraged but rather spend time preparing for the positive answers as well as improving your chances of getting an interview.