It's because with documentaries, the target audience is someone who is expecting to get some semblance of truth from the thing. But the issue is that given two documentaries, A and B, if A is more narratively/cinematically/etc titillating, it will get produced over B. Which means that the cost function of engagement will drive a documentary right up to the constraint of "not lying" as it can possibly go.
If you really care about accuracy, don't watch documentaries. And please if you are one of my friends stop recommending that I watch them to get informed about something.