This is similar to what modern-day smartphones do: they encrypt your data so that it cannot be taken at rest, but assuming that you can hijack the OS you can still read it. Also, BitLocker does have a much more stronger mode where you are actually required to enter a PIN before you reach the regular lock screen that is physically uncrackable*. In other words, this is the state if you have a) done nothing and your device is eligible for automatic encryption or b) you actively selected "TPM Only" despite clear warnings that it will be weaker than PIN.

Much more in depth info: https://news.ycombinator.com/item?id=25810639

* Unless you have cracked AES