If companies don't sell shares, then there's no stock market. If there's no stock market, then there's no rate of return for stocks. If there's no rate of return for stocks, then stocks aren't an inherently superior investment to bonds, cash, or goods.

If someone founds a company that's 100% owned by that one person, then you (as an outside investor) aren't partaking in any of the value that that business creates. Let's say you want part of the ownership of that company. Companies don't just give out shares of ownership because they feel like it. Companies give out shares because they need liquidity. They can exchange fractional ownership for liquidity directly (e.g. selling shares) or indirectly (e.g. giving employees stock options instead of salary). However, shares are not the only way for companies to gain liquidity.

Ownership that is not traded has no value to investors. Ownership that is traded is traded for a reason, and must be compared to alternatives. Those alternatives are not inherently less valuable than shares.