Because that money is sitting there idle otherwise, stagnating the economy. Money is made to be spent. Hoarding it is antisocial behavior given that personal needs have been met.
It sounds like you don't know how wealth works. This is a tax on assets. Bezos wealth is from the fact that he owns a large amount of Amazon stock. It's paper wealth. He would have to sell his Amazon stock to get the money. It's not cash sitting in a bank account.
Sell it, or loan it out. There are numerous ways to (temporarily) convert financial assets to cash without just selling them. That could be used to invest in the local economy, for example, but again to my point, it's not. And all of that is moot anyway because for ordinary citizens, your income is taxed based on your total assets (including stock), not just on your actual cash money that you receive each year.