What keeps founders in startup after vesting?

Suppose 2 founders started a company. After 4 years and full vesting one founder decides to leave. Then after 2 more years company successfully sold and both receive the same value. That looks unfair. So why founders are compelled to stay in a company even if they receive the same value as if they not?

There are several reasons to stay, and why the founder who leaves could be at a loss:

• The startup could raise another round of financing in which the founder that left could be significantly diluted.
• The founder that stays would likely get equity grants to ensure they're compensated for staying on board the company (ex: look at Aaron Levie of Box). Thus, getting more equity in the company (and a higher payout upon acquisition or IPO).

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).