Because dash cams with microphones cost almost $30 at Walmart, and who can afford that.
Yep. It's not the cost, it's police unions fighting their use.
And when they do have to use them, they fight to be able to turn them on and off.
This way, they can "forget" to turn them on when convenient.
Body and dash cams have saved more cops than they have hung out to dry. In some cases they have established that the politically connected person involved is lying. And in other cases, they might have mitigated the behavior of the cop.
They also make for some good Youtube videos.