Hmmm, this stumped me for a while. Your argument makes complete sense, but then why do I need drivers for my OS to access the NIC, when it should be able to use the same API exposed to BIOS? So I looked it up: https://en.wikipedia.org/wiki/Preboot_Execution_Environment

It's fairly light on details, but my interpretation is that for a NIC to support PXE boot it has to have firmware that exposes a standardized API so that the boot loader can functions as a DHCP client and a TFTP client. As far as I can tell, the NIC doesn't have to provide some standardized method for general access, just those two things. The NIC could even have a completely different, completely separate API that it expects the OS to use, and just has some firmware that acts as a shim to expose to the boot loader the PXE client API.

Assuming this is true, Intel ME could access the NIC using the PXE boot API, but this only gives it the ability to act as a DHCP client and TFTP client, not arbitrary networking operations.

I could be completely wrong about that, though. This is, at best, an educated guess.

The bios and uefi obviously can also access the hard disk, otherwise you can’t boot. The OS still has its own driver though. That’s because (at least the BIOS version) is optimized for a small code size and a limited environment, while in the OS you want speed and features.

pxe booting loads a lightweight os, ime can access the networking later when the full os is loaded but has generic or specific drivers for low level net access. ime doesn't use pxe though, afaik