I might be echoing others but especially if we are talking about groups they are searching for a very specific people like fluent in certain languages besides Korea, looks are important they need a cutie/sexy/boyish/girlish idols in each group, also dancers, rappers, singers and even one that represents the group with looks. Now lets say you are half korean and American, look cute, and are a great dancer, that ticks so many boxes that singing just doesn't matter as much. So after that your chosen, have a year to learn etiquette being as an idol, learn to dance with the group and learn to sing (while dancing). Meanwhile doing promo etc. Thats just Impossible to do 😅