It isn't though, not entirely. In modern history, although we, in the UK and USA, regard the history of the first and second world wars from the victors viewpoint we have more than enough access to accounts of those on the "defeated" sides.
I think the advent of mass communication , photography and investigative journalism have all but destroyed that belief. Now, nothing is believed. Older history is obviously harder to verify and corroborate but at least no one in their right mind believes that the account of the "victors" is an accurate account of what actually happened,