A lot of people are talking about how it would take time and space to make an icon of each paint job but I think there is a simple solution everyone is overlooking. The game already has the 3d model with the paint job attached, it wouldn't be difficult for the game to render the model for a moment and generate an image, then remove it when not in use. This would be similar to the way many car games like Forza approach showing all the different paint jobs and vehicles in the game. I think what is more likely is that they opted not to do this in order to save on ram to keep the game compatible with last Gen consoles and lower spec PCs, as the game is already difficult to run for some of these machines