Western cultures love a good epic that explains things. We really do.
It all really starts with the Greeks and their fixation with Homer’s Illiad and Odyssey. Of course, the Greeks saw themselves as the descendants (spiritually if not physically) of Odysseus and Achilles. They were living out that heroic tradition.
When the Romans needed to connect their own greatness to the epic past, they came up with the idea of Rome being founded by one of Odysseus’ opponents, Aeneas of Troy.
And when the British wanted to come up with a reason their island nation should be great, they latched onto both Homer and Virgil and created their own epic around a character named Brutus, one of Aeneas’ companions. This story first appeared in Historia Brittonum in the 9th century, and it was still being repeated when Geoffrey of Monmouth wrote three centuries later.
From Brutus, they got to the mythical king Arthur; and believe it or not, every British monarch since Edward I has claimed to be a descendant of Arthur. (Of course, Arthur probably is based on a historical person who would have lived around the time of the beginning of the 5th century CE – roughly the same time as St. Patrick and a number of other historio-mythical characters.)
Culturally, we have a longing to be a part of something, and that something is not just the Christian tradition Europe has bounced around for the past 1500 years or so. Europe reframed the Christian narrative within this great epic framework – what I am calling the Helleno-Romance-Briton Historical Epic because it is possibly the most awkward term ever devised.
I haven’t fully processed the idea yet, and it will probably come to nothing but it is lurking around the edges of my brain, so I figured I would write it out and let it gestate a bit.