It is because an examiner isn't involved in a night rating.
TCs (Form SRG1100 - Temporary Certificate of Licence Privileges for Ratings or Certificates) can only be issued and signed by holders of valid Part-FCL Examiner Certificates, who are qualified to conduct the relevant Skill Test, Proficiency Check, or Assessment of Competence.
An issuing examiner is responsible for ensuring that an applicant
has met all training, testing and regulatory requirements for the Part-FCL Rating or Certificate applied for,
is not seeking a TC in relation to initial issue of a licence
is not subject to any CAA licensing action, including the suspension or restriction of privileges
Examiners must bear in mind that issue of TCs is not mandatory and that, even where criteria for issue have been satisfied, they retain final discretion over whether to issue to a particular individual. Any TC issued contrary to these criteria will be invalid, and any examiner involved in such issue may themselves be investigated and subject to enforcement action, depending on the circumstances.