Right, and more to the point there's nothing special about a TPM design-wise. It's actually a very odd kind of chip that only really exists due to the unique political and market requirements of the PC industry. If you look at vertically integrated platforms like Apple's, or the games consoles, or smartphones, there's no TPM. There are subsystems that do similar things, but none of them follow the TPM design specs.
Even Intel abandoned it when designing SGX. SGX doesn't involve a TPM at any point.
So for a GPU vendor there's no reason to introduce the additional complexity of handshaking with a TPM. Blowing a private key into some eFuses at the factory is relatively easy, add a RAM encryption engine on top and you're already providing better security than what a TPM provides.
