Why journalists can use anonymous sources but historians cannot

Why can journalists use anonymous sources when historians generally cannot?

A journalist can personally assess a protected source while reporting a current event, but a historian must provide an evidence trail that later researchers can independently check.

Journalists often cover events while they are unfolding, when the public needs information and an informed source may speak only if protected from professional or personal consequences. Anonymity can make it easier for that person to speak honestly. The reporter can meet the source, discuss and substantiate the evidence in real time, and decide whether to trust the account even when the public cannot see the source's identity. A joke about ancient Greek predecessors receiving information in dreams introduces the modern historian's obligation to provide evidence that others can check. Another historian should theoretically be able to follow the same list of sources and substantiate at least the evidentiary part of an argument and analysis. An anonymous source whose identity and context cannot be recovered breaks that chain.