I mean, I was least half assuming that vehicles are being done by pairs because they are about half the volume of mechs. Which makes a ton of difference in 3D printing and presume also injection moulding in terms of production cost - and one also assumes that you would, like, fit eight vehicles in the same size box as four mechs; which would, in theory, standardise your packaging box sizes?
That there is an assumption people will use vehicles always in at least pairs seems to be a more unlikely consideration of the two, since the latter reason is basically down to game mechanics and the former potentially involves real-world money and one feels that is more convincing arguement.