Because it takes too long. Even for a little air taxi operator we get literally hundreds of applications, mostly from people asking for FO positions (we operate single crew) or people who can't be bothered to check our minimum hours requirements and are unemployable based on that.
Recently I sent a one line reply about our minimum hours and was bombarded with abusive emails because I did not put enough care and attention into my reply.
Words of advice. Apply for a job that may be available, addressed to the company you are applying to, if you just SPAM the airlines with poorly merged emails expect no replies.
Treat airlines as you would like to be treated, en-masse (I know some people target their emails and mailings well) and the operators may not have to deal with so many waste of time mailings.
Rant over.
Phil