I was in an accident once. Another driver did a blind right turn into the center lane in front of me, past a stopped bus in the curb lane. There was no issue assigning fault because the bus driver had a dash cam with the ability to record on demand. If things worked like you would like, that wouldn’t be the case.

There are perfectly valid reasons to record dash cam footage other than an accident you are personally a part of. And any issues you have with dash cams could be almost as easily, if not more easily, done with another camera-based device.