People do. All the time. Trouble is that it is not always true.When there's a terrorist attack, why do the news stories always say that [group] "claimed responsibility" for it? Why does nobody ever just say "It was us, we did it"?
News reportage tends to say "claimed responsibility" because that is exactly what it is, a claim. Once there's external verification (rather than just someone saying so) then the news reportage changes.