Here's why...
An iPhone (or iPad) can have only one active WiFi connection.
Wireless CarPlay establishes an ad-hoc (i.e. "point-to-point") WiFi network between the iPhone and the Infotainment system. While this ad-hoc network is active, the iPhone cannot connect to another WiFi infrastructure network - such as a Starlink router. Bluetooth is initially used to establish the WiFi connection between the devices
By contrast, while using a wired CarPlay connection between your iPhone and the Infotainment system, the iPhone's WiFi connection is available - and can connect to an infrastructure WiFi network.