Would like to expand this to singers too. Why do non-US singers adopt an American accent to sing with, if it’s not how they speak?
Timely and relevant example: big hair man from Hundred Reasons sings ‘festerr’ /'fεːstəɹ/ rather than ‘faster’. He is from Surrey: this is unacceptable.
Normal hair man from Hell Is For Heroes sings ‘faster’ /'fɑːstə/, like a person from London where he is from. This is correct.
Although: on HR’s third allbum, there was a song called ‘The Chance’, which hair man sang as ‘chance’ /tʃɑːns/ rather than ‘chence’ /tʃεːns/ as he had on earlier records. Presumably this was him going back to his roots, Rocky-style, with pronunciation.
I got the feeling that a lot of singers from SW London (shout out The Peel/Gravity DIP) were putting on an accent because they felt they had to, and were singing worse as a result.