As a former economics major with a specialization in international trade, I agree with you in theory. It is a basic economic tenet that production should occur where there is the most natural economic advantage. Your example of natural gas versus oranges or strawberries is a good example, because large quantities of natural gas results in production economies of scale that outweigh the added transportation costs, or the scarcity of the product elsewhere. The same applies to the oranges and strawberries. Not having to heat huge greenhouses more than offsets the transportation costs from Florida and California.
The theory is that everyone gains the most advantage by allowing the free flow of goods and services for natural advantage or the ability to produce in larger economies of scale. But the reality is more complex. First, the theory assumes a level playing field. That exists more or less between Canada and the USA, at least to the extent it ever can between two different countries. It does not exist between China and any other country. Aside from much lower labour costs, which can be a valid natural advantage, they also had much less restrictive labour and production standards and many artificial barriers to open trade internally than the countries with which they were supposedly "competing".
It is unfortunately not even close to the level playing field the basic economic theory assumes. There are short-term advantages, such as the $5 T-shirt or increased corporate profits. We all potentially benefit from those in the short-term. However, I would posit that the quality of that $5 T-shirt is often not the same as what was originally produced by the clothing mills of the east a few decades ago, IME!!
Pure economic advantages are not, nor should be in a more complex economic world, the only determinants. Strategic planning and some measure of increased self-sufficiency can also be laudable goals that are taken into consideration in more complex economic models, once you graduate beyond Economics 101. One of the biggest impediments to the reaching of modern "free-trade" agreements is negotiating through the maze of each country's internal, artificial barriers, such as lax labour standards, government subsidies and tariffs or other trade barriers.
We are seeing today some of the implications of looking only at lowest cost or largest profit. The massive diversity of manufacturing and the ability of the USA to quickly retool to necessary wartime production, was one of the reasons the country was able to quickly go to war and have its presence massively felt following Pearl Harbor, despite the loss of much of its Pacific Fleet.
In this time of peril today, we are seeing examples of both small and large companies, on both sides of the border, scrambling to find ways to quickly produce quantities of items in desperately short supply, such as masks, gowns, gloves and ventilators. Some of those examples are down-rightly innovative and show that we have not lost the ability to be creative. Kudos to them, especially those that are not using it as a method to price gouge at our time of greatest need!
The bigger part of the problem is that worldwide demand has overwhelmed the capability of the manufacturers to produce sufficient quantities. But many of those facilities are in China. Many of them were shut down while that country was fighting the initial outbreak of the pandemic. That has exacerbated the supply issue. The transfer out of the technology and the ability to produce much of it ourselves, together with the lack of diversified manufacturing locations in the world, has hampered the West's ability to quickly respond and help solve the issue ourselves.
You can tell by the length of this "dissertation" that I have way too much time on my hands while isolated here in my home, observing physical distancing. My apologies to those of you looking for a short read!!
However, maybe there is a longer-term lesson to be learned here, beyond just the classic economic one!