Why do rich people invest when they are already rich?

If you live in the US, you are in a republic - governed by laws which recognize the right to property. Most of the investment in the US is actually putting capital into companies to enable them to grow. If they are not making more money, they are losing money. It doesn't take very long to waste a couple million bucks if you don't invest it properly. (and a couple mil is probably 10 years or so of a comfortable life style today, given a reasonable earning capability for a college graduate.)