I doubt any billionaires have that much money “sitting in a bank”.
Most wealth is non-liquid. For example, if you found a company that becomes massive, and you maintain a controlling share, then you could be a billionaire on paper while having no real money to spend – the only way to turn that into “real” money would be to sell shares in the company, and thus lose control of it. If the company is doing good work, it could be better to retain control and act through the company, by ensuring that it pays employees good wages to do good work for the benefit of society. This is not completely incompatible with profit in theory, though in practice…yeah. I’m not sure if there are any such billionaires in the world today.
The real problem is more fundamental to the economy, in that it fairly consistently rewards bad behavior.
Larry Page basically became a billionaire overnight when Google went public. I don’t recall Page or Google doing anything especially evil or exploitative before that, though their success was certainly built in an unsustainable economic bubble.
If Amazon didn’t treat its employees like shit and poison the entire economy, then Bezos could probably still be a billionaire and I wouldn’t necessarily hold that against him.