$9 million is not a lot by contemporary politician standards, so I bet he’s also got a lot hidden away elsewhere or dispersed through his family members.
That's the key. Hunter's Burisma scam wasn't just to make Hunter rich. Hunter gets to keep a bunch for sure, but you can guarantee Joe got his cut out of it too. It's the oldest Mafia money hiding trick in the book, the boss on paper doesn't own anything but his family members are all mysteriously wealthy. It's how Huge Chavez's daughter came to be worth $4.2 billion.
Ask yourself how Elijah Cumming's wife got rich despite never having anything other than menial jobs. How does Chelsea Clinton afford a $10 million apartment despite never having had a real job.
It's a standard tactic for crooked politicians to funnel money to family members so they can keep themselves looking clean. A good portion of that Burisma money is for Joe, Hunter is just the bag man.