The biggest NVME drive I can find is 2TB. Apple might be offering a 4TB option, but isn't that less good? You lose all your data at once instead vs losing half.

I doubt they are hurting for real estate with that screen size, it might just be better to add another M.2 spot on the board. Then they wouldn't have to push manufacturing limits to source a 4TB drive and can get higher yield 2TB drives.