Earlier quoted context omitted.
You can make this same argument about a great number of things. Why is steel any more critical than food or vaccines or the like? Indeed we got caught short of vaccines recently, and had some nontrivial consideration of running a military op against NATO member over it.
Steel comes from iron, which you can't grow. It's more critical than food, which you can grow. Same for vaccines.
Right, and where is the iron coming from in the scenario where we can't import steel?