Why does the Canadian Government not accept covid-19 results from Canadians re-entering the Country?

So a infected guy gets on a jet and flies 10 hours to Canada.  Infects 149 people on the plan.  His test results a positive and he is stopped at the boarder.  Problem is then 149 infected people enter the country with negative tests.

Yes they do-

If a traveller presents a negative covid test at least 72 hours before arrival, they are permitted to enter the country, by they, I mean the health officials doing the screening at the airports as well as customs officers.

Between the test, and the travel, people can still become infected.  Showing an infection can take time.  Taking a few days to see, and make sure is prudent.