Why cannot the machine be woken directly via WOL after being fully powered off?

Why cannot the machine be woken directly via WOL after being fully powered off and reconnected to power?

This function is not achievable. By hardware design, all WOL configuration states will be cleared once the machine loses power. To elaborate: When a wake-up command is sent, the wake signal reaches the BIOS chip. The BIOS can only configure the PCH after the system boots. All configurations are erased upon power loss. If an EC is integrated, this capability can be implemented via an EC firmware update.

The Wake on LAN feature relies on a continuously powered standby power rail (+5VSB) to retain wake configuration registers on the network adapter and PCH. When the PC performs a normal shutdown, the main power is cut off while +5VSB remains active, so the Wake on LAN settings are preserved.

Once the power adapter is unplugged or the system undergoes a complete power cut, +5VSB is also removed. This leads to two critical outcomes:

1. Lost settings: WoL configuration stored in the registers of the network adapter or PCH gets cleared.
2. State reset: When the system recovers from the fully powered-off state (G3), the PCH and network adapter require reinitialization. This process normally needs a full power-on self-test (POST) to complete.