Because if that trade partner unilaterally shuts off trade with you due to a dispute or war, then you are hosed. They can probably find other buyers for their steel, but you will have to pay a lot more, if you can find enough at all.
Tariffs, which each country can apply, limit any trade it doesn’t believe are in its interest
This problem seems completely unrealistic because then you simply buy through the countries they haven't banned? What you're suggesting will only work if said country is a global superpower that can bully every country to tow the line. At the end of the day, no country can produce all the "critical" things it needs so this national security defence of restrictions on trade seems like a very unwise insurance to buy in almost all cases except maybe your millitary supply chain.
Very strange watching you call the current situation “completely unrealistic.” Then again reality is not realistic I suppose.