I have a suspicion that the mainstream media writers purposely under-resarch their stories so they can get the story out as fast as possible & minimize liability for libel.
Libel laws say that if you write something about someone that you believe COULD be true, it's not libel.
So imagine you are a writer who discovers a sensationalized claim you really want to print. But it could be completely false.
If you fact-check it, and it turns out to be false, now you know it's false. You could be liable for libel if you decide to print it.
However, if you don't fact-check it, you get to print the claim because you sincerely believed it could be true. Also, you get the advantage of getting the story out quicker, and you can always change it later if someone complains.
As it happens, the relevant case law is New York Times Co vs. Sullivan. According to that case, libel liability still applies when the information published is both untrue, and was published "with reckless disregard of whether it was false or not."
In other words, a policy of publishing without investigation would not limit libel liability. If the information is false, and the publishers did not have a good-faith belief that it is true, then they might still be held liable.
Libel laws say that if you write something about someone that you believe COULD be true, it's not libel.
So imagine you are a writer who discovers a sensationalized claim you really want to print. But it could be completely false.
If you fact-check it, and it turns out to be false, now you know it's false. You could be liable for libel if you decide to print it.
However, if you don't fact-check it, you get to print the claim because you sincerely believed it could be true. Also, you get the advantage of getting the story out quicker, and you can always change it later if someone complains.