It's a marketting thing, why don;t we have Singer, Hillman, Morris, Humber, Austin, Riley .....
Many of the GM cars on sale accross the world have their development done in one or more country. The reason that Opel, Vauxhall, Holden etc are not sold in the US is the same reason GM do not market Saturn, Geo, Pontiac, Buick in the UK.
Many models are based on standards based chassis components etc. lookup the GM Epsilon platform for interest.
Some cross over models do exist, the US Chevrolet Cavalier is related to the Opel Ascona as such.
The Saturn range are to some degree based on the Vectra and Saab ranges.
To create a new market for a brand may only compete with existing models, there is also the notion that different countries have different desires/needs/wants.
I'm sure there are a few grey imports however.....