“Try shipping something / anything in a parcel to China.”
China is EASY compared to Canada.
I used to somewhat regularly ship specialized test equipment into Canada. Nightmare.
Selling to Canada is almost impossible without a distributor. Excessive duties duty destroy even small sales. UK is similar. I've been been able to send non-commercial samples to China without too much trouble, except for expensive shipping.
Essentially, the whole world has been able to easily sell into the US market, but they have in turn used tariffs, regulations, and shipping fees to prevent US companies from easily selling into their markets. The better solution would be for these countries to voluntarily drop their restrictions - but since that isn't going to happen, cutting them off from easy access to the US consumer is the only short-term fix.