COPRQuestion said:
It's a good question from the OP, I understand why it's not possible to board a plane without the PRTD but some would argue as to why a PRTD is required in the first place.
Time.
When a commercial flight comes in, they have hundreds of people all at once. It's hard to do a good investigation quickly, so a lot of people have been able to enter when they shouldn't. Background checks are generally done based on name and birthdate - if there's a match, it's often necessary to call the country with the record to make sure the person matches. Checking all the databases takes time.
eTAs and PRTDs give the government time to do their investigations. As a PR coming across the border, they don't know how many days you were and weren't in the country. Canada doesn't yet have strict exit controls (though they are getting better). Requiring you to apply for a new PR card or PRTD ensures that you have to fill out the application to give them the information they need for their investigation, and that they have time to do the investigation. The eTA helps ensure that PRs don't just sneak in using a visa-exempt passport. If a PR applies for an eTA, they have the information to catch the person, and the PR has to lie in the application.
There was a lot of abuse with the PR program, and people getting citizenship who shouldn't have. Chinese nationals in particular were abusing the system - doing things like driving down to the US, then flying to China. When they came back several years later, they had fake documents showing they were "in Canada" for years, and could immediately get citizenship. That is one of the reasons for information sharing with the US - Canada now gets exit records from the US and outbound planes, so they can better detect that kind of fraud.
They lose money on the PRTD.
