American accents are often so unintelligible as to need subtitles - I can't watch many Hollywood films without them, especially if the dialogue is quiet or secondary to something happening on-screen.
The issue here of course being that "big film" is made in Hollywood and country-specific film industries are often much smaller and tend to stay far away from big-budget genres, rather than that we should try and find a globally understandable accent. Hollywood having taken over the globe is not really a good thing for anything except economies of scale.
They are unintelligible to you due to the fact you never hear them. And once they become intelligible you'll have a larger amount of material to consume and you'll be even happier.
British people can understand American variations, e.g. 'sidewalk', the pronunciation of the last letter in the alphabet and 'candy'. But there are a surprising amount of Americans that simply do not understand the original British English variants, e.g. 'pavement', 'zed', 'sweets'.
This even applies to those few Americans that have passports and travel to the UK. This lack of initiative when it comes to understanding language is possibly due to how American words have to be spelt out and explained. In original British English context is good enough. So a 'bin' will suffice for a receptacle to place your rubbish in, whereas, in America, you have to have a 'waste paper basket'.
This latter point is something that British people do laugh about, e.g.:
Americans Don't Understand English - The Jonathan Ross Show
I have the same problem with many American movie actors, and I'm an American. Watch a movie from the 1940s. There was a standard of enunciation that seems to have disappeared. But there are plenty of exceptions, actors who take that part of their craft seriously: James Spader for example. Every word of every whisper is easily understood.
The issue here of course being that "big film" is made in Hollywood and country-specific film industries are often much smaller and tend to stay far away from big-budget genres, rather than that we should try and find a globally understandable accent. Hollywood having taken over the globe is not really a good thing for anything except economies of scale.