Why can't you exercise and move on? Are you an early employee with lots of equity at a low strike price and a low enough salary that you can't pay the tax to e

Why can't you exercise and move on? Are you an early employee with lots of equity at a low strike price and a low enough salary that you can't pay the tax to exercise? Are you risk averse and don't want to pay to exercise until you know there will be a payoff?