The manufacturer then signs the public portion of that TPM key, creating the ability for everyone to assert that said key was generated internal to their hardware (and thus couldn't be used by an emulator).
You yourself could also sign the public portion of the TPM key, or even generate a new one and sign it, but that wouldn't affect the perverse incentive generated by the manufacturer's assertion. It would just enable you to assert that you trust the TPM key is internal to the TPM without trusting the manufacturer's records.
We're dealing with something like the dual of software signing here.