Why do you need a separate PIN anyway? Shouldn't your Windows password be enough? Having to enter two different codes makes it unlikely a majority would use the system. I would be surprised if iOS or Android required a separate PIN for encryption.
But Windows doesn't need the OS to decrypt a BitLocker volume anyway because the bootloader can do it... otherwise how could a FDE disk ever boot in the first place?
Unless I want to change it... or have multiple OSes/partitions where I need the entire disk encrypted.
Plus if you ever needed to change or reset your password, that complicates the encryption.