They don't. We just lose our hearing.
Accents are caused by tones in your voice, the same is with singing so this tends to mute accents.
I can generally tell the difference between American and British singers though, but not always because it depends on what style of music they sing.
They don't. Professional singing involves doing certain things you don't normally do when speaking (placing your voice a certain way, lifting your soft palette, etc) so sometimes this downplays an accent
Not all of them but pop music is really an American format so they copy the American singers they've always listened to.
Because it's easier to speak American.
I just always wondered
