It all has to do with "World Cars" meaning they are styling salable throughout the world. Companies don't want local only vehicles when there is a potential world market.
The rot started with the re-badged ISUZU Gemini and the Mitsubishi takeover of Chrysler. Early Holden styling was US as Byron stated but the EJ was actually German (Opel) based design.
This is something they went back to with the Opel Commodore. All said and done you need to look to 1974/76 for the switch to World Car designs and thinking.

Sheep design them and sheep keep buying them~!