Possibly among other issues, because if you grant options to an employee that are below the fair market value of the company, you've just created immediately taxable income for the employee.
As far as I understand, granting an option now with a currently "fair" strike price which "vests" in the future (but only if the person is still employed), does not create a taxable event at the time of vesting. However, granting an option in the future at the exact same strike price at the exact same time, creates a taxable event.
So my understanding is that option vesting is "simply" tax-preferred.