This is getting off-topic from the OP's problem.
The point of TPM is two-fold. An encrypted drive isn't necessary.
The TPM needs to verify the boot code by checking it's signature. The goal is some assurance of no tampering. The second is for the boot code to verify that it was indeed launched from a trusted platform eg. TPM. In other words, someone did not yank the drive from the machine and attempt to boot it on another.
To dumb this down ... TPM != bitllocker.
TPM is a bit of a chicken-and-egg conundrum. I'd like to load my own keys into TPM so I can sign my drive's boot code. But if I can load my own keys, someone else might do the same using their keys and altered boot code.