You aren't the first one to think of this

The reason it isn't done is for several reasons. There are actually fairly significant duties payable on a new or fairly new car... I believe the car has to be 10 years or older before it starts to look feasible.
Transport Canada restricts certain cars from being imported. I think now this might just be the 10 year thing from above but they also have requirements on changing the vehicle to Canadian standards... many models have very slight differences but these changes can add up.
Finally, depending on how the company is structured with respect to US and Canadian divisions you warranty is most likely void. This is a decent chunk of the price of the car that you are throwing away.
Most of these same rules apply going in the opposite direction as in the 90s we were buying cars for much cheaper than those south of the border. There was no flood of them buying cars from Canada. The exception would be the reserves that straddle the border... they have the luxury of buying the car in whatever country they choose I think.