Where does that initial investment come from?
Where does any money come from? It's always recirculating. Double taxation is a foolish argument.
Any transfer of money is taxed with very few exceptions that only allow for small limited transfers.
When you invest money, you're buying a stake in a business so you are an owner. When they make money, and you sell your stake, you've now completed a purchase and a sale. The capital gains is being generous because it only taxes you on any profit from the sale, unlike a normal sales tax where you would pay a tax on the initial purchase instead.