Because Apple Silicon processor acts as a storage controller, and it interfaces directly with NANDs.
It's an interesting design decision, but painful for end users because there are no standard M.2-like interface for NAND access.
I get it, and they are of course welcome to do all they want with that onboard storage. All I’m asking for is one internal storage slot. Not even for primary storage, don’t give it any special privileges, just let me keep media on it.
The fact that people are willing to go to this length at the tiniest sign of encouragement tells me I’m not alone.