The real-world reason is that, although the filmmakers tested Eve with an American accent, they decided to go with her natural accent because they felt it helped sell her as a scientist. To Americans, English accent = smart.
That is so sad.
Did Americans become dumber between the 1980s and 2010s?
This has been the case since well before the 1980s. In film certain accents equal "scientist". British English (of a certain type) and German are two of them.