The Canada-U.S. border is 8,891 kilometres .
There are 119 border crossings.
If you refuse these people at the border crossings they will  just cross surreptitiously somewhere along the border.
Let them in, vet them as quickly as possible, and send them back, if they do not qualify. And it appears the majority do not qualify.
You would need  a lot of border patrol troops to man 8,891 km. It would be a huge cost. Much more than vetting, and rejecting.