That's a "feature" of the vehicle's infotainment system. Some of the vehicles I've driven run Music the moment I connect my phone. Others are more respectful and wait for me to tell them to play something.

The aftermarket CarPlay adapter I installed in our Toyota is in the latter group. When I climb into the car, the CarPlay UI pops up on the vehicle's screen, ready for me to ask it to do something. Until I do, it does nothing other than show the homescreen with icons on it. A Ford I rented would fire up Music the second I plugged my phone in.

At least on the android side, that's a setting. Not sure about the defaults though