Well, because.... Well, I've never inquired. That would be the short answer.
I would suggest to you that if you did inquire, you would probably be disappointed by the results of your inquiry. The Access to Information Act applies to all government institutions. Although the Privy Council Office is a government institution, I don't believe the Prime Minister's Office is a government institution within the meaning of the Access to Information Act. The government institutions are listed in schedule I of the act. Although the list is not exhaustive, I don't think it applies, but that would be a question better addressed to the Information Commissioner.