Because they don't have to, yet. It's an "if", and it's a big "if".
IMO, the US should publicly commit to a solid order for a very large number of the weapon systems Taiwan and Ukraine would likely need in an extended double conflict. We saw in COVID (with things like tests) that "we need a billion of X, but we'll dribble out the order in much smaller quantities" tends to make industry leery of evaporating demand after making big capital investments.
IMO, the US should publicly commit to a solid order for a very large number of the weapon systems Taiwan and Ukraine would likely need in an extended double conflict. We saw in COVID (with things like tests) that "we need a billion of X, but we'll dribble out the order in much smaller quantities" tends to make industry leery of evaporating demand after making big capital investments.