Why does CarPlay switch to phone from mapping app in CarPlay when making or receiving a phone call.

Frustrating when a call comes in and it automatically takes your map away (Google Maps, Maps, etc.) on the display and defaults to the call. Not sure why it would not minimize the call and keep your map up. If you travel a lot for business and field a lot of calls, this really gets to you. I think Apple could understand this and allow the map app to stay up while on a phone call without the need to manually switch back.