There’s no evidence it was done “all the time”; the most logical explanation is that all these figures were based on real people, even if their exploits and histories were exaggerated and mythologized over time, just as we see with figures like George Washington.
The idea that some random historian or politician simply convinced everyone his fiction story was true and central to their identity - and it worked time after time AND everyone else bought into it - is clearly absurd. Just pointless cynicism with no basis in actual human psychology.
> The idea that some random historian or politician simply convinced everyone his fiction story was true and central to their identity - and it worked time after time AND everyone else bought into it - is clearly absurd.
It's really not. It's exactly how Romans changed from their Roman worship to christianity. It's how they converted the pagans. It's how Muslims were able to rewrite Christianity into their own religion. It's how Mormons did the same.
This is such a fundamental part of humanity, it happens all over the place.
Having a charismatic and/or politically powerful person say "This is how things are, believe it or die" does wonders to spread belief.