Perhaps the biggest unsolved mystery of human history.
@jojo69 proposed an answer to that. May be it's correct.
I wonder if AI may be able to provide us with an answer one day. I think there are several gaps in our understanding, either put there deliberately or just blind spots that evolution has put there to allow us to continue. However, even the very limited LLMs that we're currently being served are heavily tinkered with because their owners become unhappy with the output.
The current AI Chat Bots are centralised one that are controlled by the companies that created it. AI will be of little to no use in answering this question if it keep following it's current pattern.