It is written by PR people, wire services and politicos. The journalist simply assembles the ready-made components into the lowest energy configuration.
A lot of the time, it's not even sent to the journalist in question. It's just 'someone else reported on something, so every site in the same niche rewrites the story in their own words'.
That's why every single 'controversy' and news story ends up on every media site under the sun.
Yes, it's definitely the case for a large amount of stories. Paul Graham has written on the phenomenon and how taking advantage of it helped ViaWeb. [0]
Journalists talk to those supposedly "too busy" people. You'll find plenty of experts quoted in your average news article.
Plenty of journalists have experience in the fields they cover (check out the SCOTUSblog's publisher's bio, for example: http://www.scotusblog.com/author/tom-goldstein/) and the longer they cover a particular beat the more info they get about the field they're covering.
