Except that it can't actually do that, because x86 DRTM doesn't remove SMM handlers installed by the system firmware, ACPI tables also remain resident and could be changed to contain malicious code, etc.
While what you said is technically correct, it is by design and any compromised firmware can do as it pleases before the DRTM event at the cost of getting caught and having the device fail attestation or not be able to access encrypted data (depending on what policy is layered on top of DRTM itself as it is just a security primitive). By having PCRs get reset during the DRTM event secrets are much more reliably able to be sealed to specific PCR values.
[1] https://blog.invisiblethings.org/papers/2015/x86_harmful.pdf...