because pilots' actions lead directly to loss of life and property damage if they screw up?
See http://www.washingtonpost.com/wp-dyn/content/article/2007/01... for the story about how the pilots union hated the retirement age limit until a bunch of younger pilots started joining the union. All of a sudden the upstarts had the majority vote and lo and behold, the age limits are imposed.
Certification of pilots is not imposed out of a desire to assure safety; it's because the lawmakers are lobbied by the pilots themselves.
Journalists would also love to be licensed, that way that upstart rapscallion Drudge and those dastardly blogs would stop eating into their business.
Ask Richard Jewell if misguided/malicious journalists matter.
journalists do have certifications and credentials that prove their worth. its a lack of care on the part of the industry and a lack of care in the consumer base that is causing a problem.
i want the best pilot to fly my plane, because if they mess up, i die. i have a lower barrier of entry for where my information comes from, because if they mess up, i might look dumb, might make a few mistakes but i'm almost definitely not going to die.
For example, journalists may give credence to alternative medicine.
just because i suggest that you go jump off a bridge next month doesn't mean that its my fault for you doing so.
it depends on a lot more than simply "who you are".