Because we’re not interested in mandating as givens the existence of the exact cultural groups, systems of government, national boundaries, historical faiths, etc that existed in the aftermath of Rome’s collapse.
The idea of this being a world where Scandinavian contact with North America might have started earlier and gone further, or where Roman policy brought a sizeable Turkish diaspora to Britain, or whatever, is exciting and fruitful to us.
And crucially, that toolbox approach also leaves room for people to simply use the People procedure to reproduce history (exactly, or with a mythic spin as you describe).