Much easier to steal a private key than liquidate a billion dollars from many banks and other illiquid sources. Especially when most of those transactions can be reversed, or otherwise raise immediate red flags.
But if you have his private key you still need to convert the Bitcoins. This isn't easy to do anonymously, as can be seen with Alexander Vinnik's arrest. If someone actually robbed Satoshi and let him live, he'd probably alert the world and the wallet would be watched like a hawk. And even if he's dead or unaware of the theft, a lot of people will notice of Satoshi's Bitcoins start being transferred.
Yes. But, it'd difficult to liquidate and spend bitcoins too without scrutiny.