Because Anglo-Irish-Aussie actors have "happened" for generations (have you ever heard of Olivier, Leigh, Oberon, Flynn, Garson, Grant, Niven, Kerr, Burton, Taylor, Lansbury, Gielgud, Richardson (and Richardson and Richardson...), Fiennes, Day-Lewis, Bonham-Carter, Leighton, Christie, Oliver, Branagh, Thomopson, Lawe, Caine, Jackson, Redgrave (and Redgrave and Redgrave...), Neeson, to merely scratch the surface?).
Because the U.S. entertainment world welcomes English speaking actors and doesn't discriminate (with the justifiable caveat of union considerations), as well they shouldn't.
Because Anglo-Irish-Aussie actors are among the best actors in the world.
Because among our biggest stars are Anglo-Irish-Aussie actors.
Because Anglo-Irish-Aussie actors are often though not always well trained, not to suggest that American actors are not.
Because films from the UK, Ireland and Australia are often popular in the U.S. and the U.S. filmgoing populace wants to see performers in these movies again and so it is natural for them to appear in movies in which Hollywood is involved.
Because Hollywood produces many films set in English speaking countries besides the U.S., including movies made from popular source material set in English speaking countries besides the U.S. including works by Anglo-Irish-Aussie playwrights and novelists, both classic and contemporary, and Anglo-Irish-Aussie actors are often though not always the best people for roles in these projects.
Because Anglo-Irish-Aussie actors are often very good at American accents.
Because Anglo-Irish-Aussie actors often win major awards in the U.S. and so Hollywood and the filmgoing public takes notice.
Because Anglo-Irish-Aussie actors often make impressions in film festivals worldwide and so Hollywood takes notice.
And finally because there is absolutely no reason in the world why Hollywood should not be interested in fostering the careers of exceptional talents, including actors, from any where in the world who have either major or burgeoning careers abroad and or are proficient in English, including but not limited to American, Irish, Australian and UK actors.
But you knew all of that.