When I search for the phrase "here's what we know," Google returns 1.28 million results and, confirming my impression, most seem to originate with news sites. First page results of note:
The San Bernardino Sun: "Here's what we know about the Kendall fire burning in San Bernardino"
The New York Times: "The Panama Papers: Here's What We Know"
Time: "Here's What We Know About the New Taliban Leader"
Los Angeles Times: "Here's what we know about Noor Salman, the widow of the Orlando gunman"
National Public Radio: "Here's What We Know About the Orlando Shooting Victims" (headline later updated to something else)
I strongly suspect that those headlines should read:
"Here's What We Think We Know About [insert subject here]"
"Here's What Someone Told Our Reporter, or We Heard at a Press Conference, or We Read in a Press Release, About [insert subject here]"
Most "news" isn't stuff the reporter sees firsthand. Some is, but most of it is just stuff the reporter has been told and believes. Which means that at best, only the reporter's source actually "knows" what actually happened. So what we're really hearing from the reporter is how trustworthy the reporter considers that source.