It’s film-makers and the spoken media to blame, mate. How often do you hear a northern accent on tv? Watch any U.S. show where a character is supposed to be working class English. They all talk with a southern based accent. They genuinely think that is how all English people speak. The B.B.C is as guilty as any. Years ago they made a film about a true wartime incident where 2 geordies (sorry for the language) stole a boat and went to France to commit sabotage. Their accents were changed to slight Yorkshire because the beeb thought nobody would understand a geordie accent. And whose fault would that be? The southern based spoken-media who for years ignored the existence of anyone north of Yorkshire. Watch any film made during the war and the accents are either cringeingly ultra posh South, or working class London, with an occasional Yorkshire, Scottish or Welsh. Things are slowly changing possibly down to the soaps. I apologise in advance if I’ve offended anyone.
I've just been to Dunfermline and understood approximately half of what he said ...
... the Yanks would be totally lost if the correct accents were used tbh.
I don't agree with your premise anyway. The Daphne character, in Frasier, is broad Manc as just one example.

