There are several reasons to stay, and why the founder who leaves could be at a loss:
This is true for key employees as well, not just for founders -- after the vesting period ends, and if they're key to the company's success, they get another vesting schedule and equity grants (for more equity on top of what they already have).
Having fully vested equity in a company that is worth zero (at the time the founder leaves) is still of zero value. By leaving, a founder has no control of the startup's outcome.
In terms of fairness to the other co-founder who sticks around to try to make the company a success, they can get more equity after the vesting is complete (with board approval, of course).