I heard cars with zany colors get stuck on the sales lot for much longer than boring colors. If that is true, customizations lower the value on the secondary market relative to factory restrained factory options, and the owners may get lowballed when trading in their cars, or will invest more time of selling as a 1st party.

If car colors were not mired in paperwork, I imagine dealers would respray them some shade of Millennial-gray, similar to realtors' M.O. when prepping homes for sale.