There's quite a few reasons this is difficult to do, application permissions and stream encoding being two of the largest ones. It helps to remember that at the end of the day, the MCU is running Linux - if you were to try to use your webcam within multiple applications on your computer at the same time, you would run into this same issue. There's a whole segment of software like Anycam on desktop platforms that's designed to get around this limitation, by duplicating the stream out of your webcam into multiple "virtual" webcams, but this does require some processing overhead.
In short, it's theoretically "possible" but not really practical to do in-car.
OK, but it all works fine when you're parked. I'm not sure why driving makes a difference. The cameras all continue to record to USB just fine even with zoom video.
I agree there are technical reasons why it might not be possible with their hardware but I'd be surprised if that was actually the case.