Most American actors do a terrible job with British accents, to the point that it becomes a meme in of itself.
My favorite is Margot Robbie's over-the-top "Brooklyn" accent for her Harley Quinn character.
It opens up American roles for them. Nobody is going to cast Brad Pitt as a Brit, an Australian, or a Kiwi. He's a fantastic actor, but they're going to use the homegrown talent. So there's no incentive for American actors to really learn another accent. Instead, we get a mountain of non-American actors doing mostly-good-but-sometimes-awful accents because that's where the money is.
Except for Irish roles, for some reason.
Brad Pitt as a northern Irishman: https://www.youtube.com/watch?v=2FjQQrxBhwQT
Brad Pitt as an Irish Traveller: https://youtu.be/9Jar2XkBboo?t=28
FTFY