It takes time, planning, zoning, money, and passing a bazillion inspections. Plus trained, knowledgeable production staff, supervisors, packaging and delivery teams.
One pharma out of the 5 I worked for as an EA had a production plant in-state, and it was 20 miles out of town. The other four were in different states, or overseas.
Many pharmas with US lab footprints are centered elsewhere. They'd have to get more clearances for international production from both their own countries, and ours, as well as patent-use clearances, which can take decades of litigation if there's a challenge.
It's not as simple as that.
M.