Because it's a social problem, not a technology problem.
At the same time, just because these instances of "missing" tape happen, does not mean that body cams and jailhouse CCTV are useless. We would not take those away. Likewise for the future drone footage
It will absolutely happen in corrupt departments, or those involving an officer with those skills and access. But data that is uploaded is infinitely harder to erase than simply turning off the camera in the first place.
Remember, oppression and invasion of privacy is still bad even if it isn't currently happening to you. If you think you can't be a target, you're sorely mistaken.