Same thing as bicycles Tony..
Merida and Giant, are two of the biggest bicycle brands in the world, and only 2 of a few that actually have their own manufacturing plants in Taiwan / China.
90% of the bikes out there are made by either Giant or Merida, to their own specs, and using their own materials.
For example, I ride a Colnago, Italian bike, designed and tested in Italy, made in the Merida factory, using Colnago's own carbon fibre and fabrication methods, however utilising Merida's machinery and workers etc.
There are 100 different 1:8 buggies out there - if everyone was made in their own unique factory, imagine the cost!