For the same reason Tesla paid SpaceX for steel, or SpaceX pays Tesla for power packs. Companies pay for services rendered. Could another company have made th

For the same reason Tesla paid SpaceX for steel, or SpaceX pays Tesla for power packs. Companies pay for services rendered. Could another company have made the tunnel cheaper? Was a tender raised? did Tesla benefit from from having a tunnel under a highway to avoid going around it/paying tolls? Those are questions remaining to be answered.