It's simply because all sponsors have strict requirements of the display of their own logos, with the right colors, backgrounds, etc. Then there's the factor that includes the deal with certain sponsors, where they likely agree on some certain area/size. Once you take those things into account, there's far less room to play with, an enthousiast doesn't have to care about that. If you take the current Yaris design, and would be able to only play with the colors used on the sponsor logos/backgrounds of it, you'd have a completely different look already.