They would if they cared to invest in it. That they've not is a policy and economic choice (possibly because they don't have the same sense of panicked urgency I see among a lot of folks around here), not evidence of some insurmountable barrier to entry.

You also jumped to a conclusion that I don't think is self-evident: that owning the production chain matters.

Assuming open weight models continue to advance, who cares? Just let the US and China expend the compute on model development.

And if they close up, then fine, do what China did and build up a domestic industry and distill as a way to get a jumpstart. We already know that's possible since China already did it.