He already has $10 billion. What's he going to do with another $10 billion? You can't spend that much in a lifetime and he's an old man. I'm guessing owning the team brings him a lot of joy and notoriety.
I know a lot of people who own businesses that could sell and retire but then nobody will know or care who they are so they sit in their office and just enjoy being the "big guy." There's definitely more to life than money, especially if you already have enough