It’s federal regulations. If the reverse camera screen doesn’t appear quickly enough, the car gets recalled and fixed. This has happened to many manufacturers.

Ford, Toyota, Lexus, etc. This notice from Tesla actually specifies the regulation:

> On affected vehicles, upon vehicle power up, a certain software configuration may prevent camera streams from being sent to the MCU for up to 11 seconds, causing a loss of rearview image for up to 11 seconds for drivers who shift into reverse during this time, which does not comply with FMVSS 111, S5.5.3.

https://www.tesla.com/support/recall-loss-of-back-up-camera-...

AFAICT, they always run in the background anyway. CarPlay is mostly just a pass through canvas rendered on the phone. The system can decide at anytime to overlay something on the CarPlay canvas.