You only collect PR cards in person when you renew them. In order to renew your PR card you need to fulfill your 730 days of residency requirements and collecting in person shows that you are at least in Canada.
My brother and father just had their PRs renewed and had to collect in person, and my father could not even collect on my brother's behalf.
If you are a newly-arrived immigrant and you have already met the immigration officer when you handed in the COPR - obviously they cannot process a card immediately for you but since they have seen you and verified your landing they can safely mail your PR card to the address you give them.