Yup doesn't make sense much. Every entry in Canada is using a PR card. On my last entry into Canada, they didn't stamp my passport on arrival, so passport alone does not help in tracking anyone's exit or entry into Canada.
They are not looking for stamps by the CBSA. They are looking for stamps from other countries.
CBSA started tracking exits out of Canada only from June 2020. So, while CBSA will have all the records of your entry into the country using your PR card, they will not have the exit records.
For example, if someone left Canada on 10-Jan-2019 to Singapore and returned only on 20-Nov-2019 but they've mentioned in the physical presence calculator that they left Canada to Singapore only on 20-September-2019 and they returned on 20-Nov-2019 (while, in reality, they actually left on 10-Jan-2019), how is IRCC going to verify that claim?
When when IRCC tries to query the CBSA's database, they will only see the entry record into Canada (20-Nov-2019). They will NOT be able to confirm when the applicant left the country. This is the reason why a lot of applicants get asked for all the pages of the passport because those pages will show the real picture (Singapore Immigration would have stamped your passport on 10-Jan-2019 with an entry stamp and by looking at this, IRCC will be able to confirm the applicant lied on their physical presence calculator).
This process will likely be decommissioned after June 2025 because the 5 year eligibility period for applicants after June 2025 will fall after June 2020 and CBSA, at that time, will have all the entry AND exit records for all trips during their eligibility period.