It somehow started around 10-15 years back, until then you could still trust the national public stations here and sometimes they really pissed some people off with their scoops, in fact they did bite the hand that fed them occasionally which was the best justification for their public funding.
The thing is, back before then, if they were lying, how would you know? If a politician changed their stance and complicit media was willing to cover for them, who would report it other than some 'nutjob' wearing a tinfoil hat in a trailer somewhere yelling at nobody? When they 'bit the hand that fed them', how much of that was carefully calculated to remove someone? (Remember when Howard Dean was put out of a race for what...? A celebratory yell? Who
told people that disqualified him? [Disclaimer - I didn't even like Howard Dean but that never sat right]).
Media
has been on the decline and ever more polarized though. It was kind of inevitable once Tim Berners Lee first typed '<html>' but a lot of it is that we can see what's going on now and have immediate recall of things that were said and done three, five or fifteen years ago.