Which display case are you talking about? The only ones I know of have been around for several years, from back in the Playing Mantis days. The license plates are from RC2, in fact they are a recycled Racing Champions deal (I didn't like them then either), so I'm sure they were not a consideration when they designed the displays.
Because if the slots were made bigger to fit the cards there would be a lot of wasted space...
And the cases would be 3 cars display cases instead of 6 car display cases.
And people would ask "Why did JL make the slots so big? What about the cars that don't come with cards?"