We know the reason...he wants to live a peaceful life with all the money he’s got. They say when you have a lot of money, your life is at risk, either from getting robbed or worse, getting killed. Satoshi doesn’t have to deal with that kind of fear despite owning a huge amount of Bitcoin because he chose to stay anonymous. He’s probably enjoying life to the fullest without even needing to work, living the best life possible, especially if he’s in good health.
What's actually Satoshi get benefit from his own invention? yes he own like 1 Million of Bitcoin, but AFAIK he never sold his coins, so how do he live peaceful life with the money he got? Satoshi actually didn't receive any benefit except he start to sell his coins.
It's actually sad that he invented something that didn't give benefit for him.
Is there any guarantee that he only owns those 1 million bitcoins and no other bitcoins besides that? No, we also have no evidence to prove that Satoshi owned at most 1 million bitcoins and he never sold any bitcoins to enjoy life. As long as Satoshi's identity remains a mystery, we know nothing about him, is he alive or dead? Is he still following bitcoin or has he given up and is enjoying life?...no one knows.
The worst thing that could happen to him was that if he were no longer in this world, it would be a pity that he would not be able to enjoy any benefits from his great invention.