Because:

1. Factories take time to build.

2. Building factories requires capital to be invested now.

3. The return-on-capital will only be obtained in the next n years.

4. But if demand goes down, we'll have much more supply than demand, leading to a cutthroat price competition, which could prevent the factory costs from ever being recouped.

No 4 is why nobody is willing to really move on new fascilities.

China is doing it but that was for merely creating tech independence.