You are missing the emphasis on the market in stock market. By buying all the stocks of a company, you are basically taking company private – you own the company but the market for its stock is gone – and the value of what you own is the book-value of the business and access to its free cashflow. You don't own an instrument that gets you multiples on that anymore. Stock market gives you a multiple on the book value.
For example, if Elon Musk owned all of Tesla he would have access to $3.7B of its annual free cash-flow or $3.3B of net income. He won't be able to afford to buy Twitter with that. But because he has Tesla stocks which gets a huge multiple on those metrics, he can pledge a fraction of his fraction of ownership in Tesla to get a huge loan with which he is able to buy Twitter (and still enjoy his ownership rights of Tesla stocks!). That's because of stock market.
For example, if Elon Musk owned all of Tesla he would have access to $3.7B of its annual free cash-flow or $3.3B of net income. He won't be able to afford to buy Twitter with that. But because he has Tesla stocks which gets a huge multiple on those metrics, he can pledge a fraction of his fraction of ownership in Tesla to get a huge loan with which he is able to buy Twitter (and still enjoy his ownership rights of Tesla stocks!). That's because of stock market.