Because it's obvious. Anna's voice is distinguishable, Sharena is the higher voice, and Veronica is the emotionless one. Ray Chase's voice is also pretty destinguishable so that's why I know that's him. No way does it sound like Ike, especially considering how he sounded in Fates.
Also, the same set of 5 voices just repeats over and over. There's no other than just those 5.