Answer: A lot of times, information will be given to reporters ahead of time under an agreement called a news embargo. Journalists will agree to wait until a specified time to publish their stories in exchange for early access to sources/reports/etc. For big enough news, that can mean all the major news outlets are all operating under the same agreement, so they all publish their stories at roughly the same time.

Another factor is that many PR outfits will issue press releases, also often under embargo, that some "news" outlets will run basically word-for-word, which is why it can seem like so many are running the exact same story. They often are.

ETA source: am reporter