I was in Taipei a few weeks ago an saw that most taxi's had dash cams - very small, very common. It makes sense to me for have these for a multitude of reasons as stated above. I don't see these being sold in the US - yet another case being behind the rest of the world - but of course we will never admit that...
I'd argue it's the opposite: It is because the relevant parts of American culture is/was ahead of many nations such that our police force and citizens are/were, on average, more honest than those in places where dash cams are more common and necessary.
You could try to counter this by saying that there's been a regression in society these days but that would only deflect the argument to a completely separate but debatable subject of its own.
The other replies ahead of mine have also already pointed out that dashcams are (and have been) available for sale in the US for quite some time. They just aren't very commonly used by the general public. Many law enforcement agencies already have them installed as standard equipment on their cruisers.