In a privately-held company, you usually can't sell your stock without approval (from the board, a shareholder vote, or some other mechanism). That generally makes it quite difficult to sell.
Dividends are the main source of value from stock in a company that doesn't plan to sell. But dividends are also determined by the board. In a closely-held company, the majority owners may also be the board, and they may prefer to leave the profits in the company or take them out a different way.
"Worthless" is an extreme characterization, but the value you receive from owning a minority amount of private stock is much less predictable and controllable than publicly-traded stock.
Dividends are the main source of value from stock in a company that doesn't plan to sell. But dividends are also determined by the board. In a closely-held company, the majority owners may also be the board, and they may prefer to leave the profits in the company or take them out a different way.
"Worthless" is an extreme characterization, but the value you receive from owning a minority amount of private stock is much less predictable and controllable than publicly-traded stock.