GoKawiilApple explains why many iPhone apps are missing from CarPlay
Apple restricts which iPhone apps can appear in CarPlay to a limited set of approved categories, which explains why apps like Gmail, Apple Fitness or Calculator don't show up even though they could technically run there. Apple has been gradually loosening these restrictions over time, expanding the range of app types allowed. Users can also check Settings > General > CarPlay to add and reorder any compatible apps they may have missed enabling.