I suspect that they do not actually contain the encryption key. It is more convenient if the disk encryption key is stored on the disk, but separately encrypted. You actually want to store the key multiple times, one for each unlock method. If the disk can be unlocked with a password, then you store the key encrypted using the password (or encrypted using the output of a key derivation function run on the typed password). If it can be unlocked with a smartcard, then you store a copy that is encrypted using a key stored in the card. When Bitlocker uses the TPM, it no doubt asks the TPM to encrypt the key and then stores that on the disk. To decrypt the disk it can ask the TPM to decrypt the stored key, which will only succeed if the TPM is in the same state that it was in when the key was encrypted.

The reason it's done this way is to allow multiple methods of accessing the disk, to allow the encryption password to be changed without having to rewrite every single sector of the disk, etc, etc. You can even “erase” the disk in one swift operation by simply erasing all copies of the key.