Earlier quoted context omitted.
Private property exists because it is an organization of things that is useful to society. That's why we heavily limit it using taxation and so forth. We allow people to own capital because it motivates them to do some types of work. If we allow people to concentrate more capital than necessary to motivate them to do that work, we lose.
There is no "we" allowing or disallowing anything. Who is this "we" and why do use suppose they have some divine insight into the ideal allocation of capital?
Shares of stock, the secondary stock market - these are also arbitrary constructs which have been created to serve some purpose.
There’s some magical thinking regarding capitalism - that it practically runs itself; that is ensures efficient allocation of capital.
Capitalism may tend to decentralize economic decision-making, and it may tend to align production incentives with actual demand. But, that doesn’t mean it’s efficient. Efficiency would mean that incentives would nearly match the minimum to ensure satisfaction of demand. What we in America have is a system which highly rewards people who control the labor of others. Our society values wealth, not work.