> you have never been able to buy American milk in canada.

That’s not really true. The US has 5% of the quota, tarrif free. Which they don’t even fully use. But, to be sure, it does get used. You absolutely can buy American milk products in Canada.

The problem is that they have to go through the processors to satisfy the processors' own quotas so by and large the meager 5% isn’t worth the operational complexity vs just buying milk locally for the processors. The US wants to be able to go directly to the grocery stores.