The problem is the differences are asymmetric. Canadian labour mobility is a lot more impactful when the pool of skilled labour is just overall smaller.

Doctors can move pretty much freely between 44 American states. Doctors can't do that in Canada, and here we need more specific kinds of doctors in rural populations and there is absolutely no reason to not have a harmonized licensing regime.

We also have non-uniform regulation on stuff like the manufacture of construction goods (for example door sizing) which can cause a big drag in trade. This might be harder to fix but it exists and is not comparable to the US market.