And satoshi don't like to crash bitcoin economy due to his greed.
There are many different logical possibilities, but the most obvious ones are these, to me:
A) Satoshi is a person
- he's dead and his heirs didn't know who he was
- he believes in bitcoin taking over the world's economy ; then it would be silly to sell his coins for a billion, when he can get a trillion and be "master of the world"
- he would have liked to sell them, but he doesn't know how to do so without giving up his anonymity: for sure, the exchange on which he sells the coins will know who he is, or to what bank account he withdraws. He was already pretty paranoid in the old days.
- he didn't bother to keep the private keys ( <-- cannot believe this)
- he never had the keys himself, because he was working for someone.
B) this last point brings us to: Satoshi is an organisation. In fact, the points are similar
- the association doesn't exist any more (for instance, the keys were in escrow, and the different signees don't agree to get together any more/are dead/are in prison /....)
- they are waiting for bitcoin to take over the world economy at which point they own a big part of it and will be the rulers of the world
- they don't know how to cash out without getting caught/found out
- on purpose, they didn't keep the keys, because they wanted to establish a system that would modify the economy/finance
- they were a group, doing this in command of a higher entity (a state, a rich family, a terrorist organisation, ...)