To defend against an attacker with physical access to an offline machine you need to verify anything that the attacker can overwrite without the encryption key. Aren't bootloader and kernel on the writable unencrypted partition?

If you have secure boot enabled, how does the attacker replace the kernel or bootloader?

Pull the drive out, insert it into his machine, replace, then insert it back.

And now the signature doesn't match, so the system doesn't boot

Which signature?

The signature that's validated by secure boot. If you don't have secure boot turned on then there's no point in verifying PCR 7, because all PCR 7 contains is the secure boot data.