You seem to think it's common to require a separate BitLocker unlock step. In reality, this is extremely rare: the vast majority of users have no idea about any of this and have BitLocker set to automatically unlock during system power on.
So this is a viable attack on many, many real-world systems. Adding a BitLocker password/PIN is a mitigation that prevents this attack.
Note that BitLocker is still very useful even in this mode: it guarantees that someone who steals your laptop can't just connect the disk to another system and read everything on it, unless they can actually extract the keys from RAM, or bypass Windows authentication - this attack allows them to do the former relatively easily.
So this is a viable attack on many, many real-world systems. Adding a BitLocker password/PIN is a mitigation that prevents this attack.
Note that BitLocker is still very useful even in this mode: it guarantees that someone who steals your laptop can't just connect the disk to another system and read everything on it, unless they can actually extract the keys from RAM, or bypass Windows authentication - this attack allows them to do the former relatively easily.