Why do American Hollywood megafilms often use non Whites to play White southern European characters?

In the slave movie Amistad the main "Spaniard" characters are played by a Mestizo Mexican(Geno Silva who plays the role of Ruiz),Mulatto Puerto Rican(John Ortiz who plays the character of Montes),Jew Argentinean(Castulo Guerra who plays the character of the Roman Catholic Spaniard priest).It upsets me to realize that the liberal Hollywood film makers are feeding Americans a false and distorted view of us true Spaniards,who by NO means resemble Mestizos,Mulattos or Jews.Sadly Spaniards are not the only southern European victims since Italians have also been the target of the false racial image portrayal as well.Why dont they use White southern Europeans actors/actress to play southern European characters?(especially when the production of the film is based on true historical facts that involves southern Europe).