Nitpick: If there was a actual historical character who was anything like "King Arthur" he lived a long time (probably about 600 years) before the first accounts of him written in the Middle Ages.
But yes, point taken. :)
Ignoring the accuracy of a tale told more than five hundred years after the event, if Arthur was real and such an important figure, why did the Venerable Bede fail to mention Arthur at all during his remarkable (for the 'dark ages') history of Britain in the ninth century? /nitpick
To the extent that Arthur could have been real, he likely would have been a warrior-leader, rather than a king, fighting against the spreading Anglo-Saxon invaders.