I have two possible explanations:
1. Sadly, he is dead and that's the possible explanation of why he didn't touch a million Bitcoin for over a decade. This is sad and I think the most legit answer.
I hope he's still around, but if he is dead and he had no one, not even relatives, to hand down the private keys, then he may have discounted the fact that Bitcoin will be worth a lot in the future, so he ignores passing the private keys to his heirs.
2. He is one in a billion man who stands strong to protect Bitcoin at all costs. His aim was the true decentralization of Bitcoin without trusting 3rd parties and for this reason, he cut himself from Bitcoin for better but left an ACE in his hands, i.e. one million Bitcoin. If the decentralization of Bitcoin gets significantly affected, then Satoshi will come out of nowhere and make a big changes with his one million Bitcoins in hand.
For a guy who created this revolutionary technology, he knows the right time for his creation, at the popularity of Bitcoin and the converted amount at his disposal if he still has the keys, He is the most important man on the planet and if his character is more like an introvert, we may not see the day he will come out openly
I can't really find the best explanation to the situation that happened around Satoshi, this is out of human's capability. Secrets are always revealed but in case of Satoshi, it still remained a secret and that's what I can't explain, that's impossible accident that somehow happened.
If ever this secret comes out, I hope I will still be around so I can follow the scenario and see the world's sentiment, somewhere in the future we will have a hint on who he is and where he is at.
Or maybe he is reading this discussion and laughing at our efforts to locate him