How come the rich don't just keep getting richer? Interesting question people?

Let's take Carlos Slim who is currently the richest person in the world and Bill Gates the second richest person in the world. My question is, when you have the kinda money that these guy's have, how come they are not trillionaires by now? Do you guy's think that there will ever be a trillionaire in our lifetime? There's old saying that goes around that says "It takes money, to make money." Can these guys not put all that cash in the bank and let it drawn in interest rates? I mean come on how much do you think you could get in interest if you have over a billion dollars in your bank account? I mean I sound a little greedy but I'm just saying if I was those guys and had that much, why stop there? Why not buy and invest into other companies and rake in the profits from those businesses? It's kinda like selling drugs, you start out with about an ounce of weed, break it down, nickle and dime bag it and then re-up with more weed and still have profits and it just keeps building up til your bring in pounds of weed.