The thing is, that if you have money, and you manage it properly, it will just grow. How much wealth is somebody allowed to accumulate? 5 million? 50 million? 500 million? 5 billion? 50 billion. Once you reach that first number, you can make a lot more money simply by making smart investments with your money.
Bill Gates made his money because he owned a lot of stock in a company that started out small and became very big. Unless you seize the stocks that he has, there isn't really much of a way to take that wealth from him. You can tax the sale of those stocks, but that means fewer people will invest because they are unable to realize any gains from investing the money. Do we want to dis-incentivize people from creating the next Microsoft, or the next Amazon, or the next Apple? The fact that you can become ultra rich creates an incentive to build these large companies up from the ground, creating new products, and creating new jobs for the rest of the people.