Because they are not always present, not convenient, and don't necessarily reflect the actual date when the content was written / updated. Maybe a file was copied and the modification time changed although the content on the page didn't. Maybe the author fixed a spelling mistake 10 years later but didn't update anything else.
I want a human-decided, human-readable date on the page for the same reason I want a human-readable headline. For a technical post, they're about equally important, and should not require examining protocol headers to determine.