by From 1930s to 1960s most actors in Hollywood are white and producers kinda prefers Europeans over White Americans and White Canadians. A cast combination of Europeans and Americans and will have different accents. So to make it more uniform... Hollywood movie actors are forced to learn Transatlantic accent. They said this accent is also easier to understand to both Americans and British so it became a standard. Another reason why its used is that this fake accent works well recorded. The limited technologies back then struggles to record more natural accents.
The transatlantic accent sounded exotic and sexy but no one talks like that in real life so it became obsolete in the 1970s.
No one talks like this in real life. Around 1970s movie producers started preferring American actors over Europeans so Most Hollywood movies from then on uses Californian accent the most. Now we can hear different accents in the US being portrayed and also foreign accents too because Hollywood becomes more diverse.
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