Why cant doentaries be more truthfull?

I think that doentaries how somehow acquired a false sense of "truth". They originally were no so distinct from fiction in the 20s-30s. A lot of doentaries explicity deal with truth as representation, and allude to the fact that all knowledge is a construction. They all vary in more ways than one, but I know few docs that claim that their truth is ever the right or the only one.