I have no special insider knowledge. I reckoned it could be turn-around time coupled with needs. A quick google suggests that in optimal conditions (all parts

I have no special insider knowledge. I reckoned it could be turn-around time coupled with needs. A quick google suggests that in optimal conditions (all parts available), it would take minimum of 2 months to assemble a large airliner plane. 6 months is suggested as a common time frame. As far as needs go, a plant would need suppliers for all pieces (over 350k by one count for one type of airbus). And you'd need a place where you can assemble the plane, with a sufficiently large runway attached. Possibly in an area where testflights would be acceptable. All in all: perhaps current suppliers are at or close to their limits, and a new facility would need a completely new supply chain. If not, I'd imagine that if the bottleneck is exclusively with the builder, a solution could be found.