After the 409a valuation of the startup gets too high, this option becomes much harder for employees, which is why you don't generally see it outside of the earliest stages.
(Disclaimer: I am not a lawyer, not your lawyer, this is not legal/tax advice, etc.)
As I understand it, it's not just the valuation, but also the tax implications. If the options (as granted) are worth more than a certain threshold (~$100K), you can't early-exercise them without losing ISO tax treatment on the value in excess of the $100K threshold. This threshold is on an annual basis, so not exercising them early means that you quadruple[0] the amount that will be eligible AMT (which is preferable to ordinary income tax).
[0] Assuming the standard vesting schedule, in which 1/4 of the shares are made available each year
Can you elaborate?