Well, presumably, when you buy a small business down the street, you are bringing your own enterprise into the equation. You can buy all the Coca-cola stock you want, it won't make you part of the business.
But that's really not the point I'm making. The point is, there is a difference between investing capital in a business and investing in stock. For the investor, the difference may be irrelevant. But for the business, the difference is huge. Only the original purchase of stock from the company brings any capital to the business. Every subsequent sale of that stock means almost nothing (outside of the fact that markets generally force public companies to worry about their own stock performance, which is also stupid).