Here is my 2 cents. 

I do not believe Satoshi ever conceived that this experiment would become this successful and that bitcoins would become this valuable. I think he only mined coins in the beginning to get things going and he most probably lost most of the private keys for those mined coins.

Remember, back then most of these coins was virtually worthless, so throwing them around and playing with them was not a big deal. Just imagine if Satoshi has to emerge now and admit that he lost all of the private keys for those coins that was mined in the early days.