They got it in their minds that inaudible dialogue is more realistic and therefore has more artistic integrity than actors giving proper theatrical performances. A good stage actor can 'whisper' and be clearly heard across the room, but that's not realistic and is out of style. So now they have actors actually whisper and everybody has to watch the show with subtitles on.

Actors should never be whispering or mumbling for real. Even an actor pretending to be drunk should be able to clearly enunciate their words while still making themselves sound drunk. I'm sure modern actors can still do this, but they're being given bad direction.