Microsoft took the idea even further with Visual Basic, one of its most successful products, but originally developed in Apple's backyard to be HyperCard for Windows. Debt financing is borrowing. Stock is the sale of equity each person who owns Microsoft stock owns a piece of Microsoft. The money isn't borrowed, which means, for example, Microsoft cannot simply pay back the original stock price plus interest and own itself once again. Borrowing is based on content the ability to borrow and pay back is based on the company's assets and cash flow.
Equity is only meaningful in the context of a market.
The public stock market is often regarded as irrational, but it is another example of a complex system that can be modeled with the mathematics of chaos and complexity, which are rational. The market, like the weather, can only be predicted in the short run, but neither one is irrational. The idea that Bill Gates' only responsibility, or even primary responsibility, is to maximize return on investment, might be arguable in the context of stockholders. But it's not what drives Bill Gates. You don't have to be a psychologist to figure out that winning motivates him, not profits.