Plenty of particular brands of vehicles have smaller production runs. But "vehicle" to the manufacturer doesn't mean "brand". It means "set of pieces and parts that can be the same or nearly so across many brands". For example, a "Cadillac" to you is a different "vehicle" from a "Chevrolet"; but to GM, the vast majority of the pieces and parts and manufacturing processes are shared. So the economy of scale to GM when building "Cadillacs" is huge even if to you it looks like "Cadillac" has a small production run.