I can think of only a few reasons why that might be.
1. He wants btc to succeed and knows spending his coins will cause a massive panic.
2. He is already rich and has no need for more money.
3. He was hired/tagged by an agency or institution and is under NDA.
4. He is sick of btc.
5. He hates attention/spotlight and does not want the leadership role.
What if the answer is even simpler? He had a car accident / heart failure and died. His wallets are lost and therefore never spent.
This is so much more reasonable and probable than "satoshi is an AI" or other crazy theories here. It's sad, but I think this might be the truth.
That could still be possible. Given the facts that nobody can really give sufficient evidence that they are the real Satoshi Nakamoto and the ongoing problems on bitcoin was still unsolved it could lead to such hypothesis that Satoshi Nakamoto really died just before or after the launch of bitcoin in his honor. There are many possibilities and this one could also be included in the list.