A 409A and the price someone is willing to pay for a private company are not the same. A 409A for an early-stage business is usually performed immediately after a financing round. This type of valuation is just a snapshot at that moment, which heavily discounts that an early-stage company can remain a going concern. An investor looking to participate in a subsequent round of financing for a private company wouldn't typically refer to a past 409A due to the high likelihood that material events have transpired since the time at which that valuation was performed. For an early-stage company, almost all events are material.
Most of the time private companies in this situation do not have earnings and are reliant upon financing. A 409A valuation is most meaningful for employees of these companies who may need to report deferred compensation on their tax returns and need a share price in order to do so.
Investors considering companies that have profitability and the ability to forecast growth and cash flow will utilize different methods that are more forward-looking.