Here's my theory about why companies like EEG and Jungiery go for the "good looks" and teeney bopper idols to market.
With the advent of technology, everyone can essentially download any CD they want. That means the vast majority of the population will be lazy and cheap--why buy when you can download?
Yet, we see that people are still buying CD's. What group does this consist mostly of? FANS..especially diehard fans who enjoy seeing some money go into their idols' pockets as they care sooo much for their idols. That's why they create star blogs, fan meetings, etc. to make the fans feel even "closer" to their idol, instigating more feelings of wanting to buy any products related to their idol to help them generate revenue (as if they didn't have enough).
If a person isn't a diehard fan, but just someone who liked an artist's music, it's most likely that they'll just download the CD for free. But if they idolize the person, they'll be more willing to buy the CD. Hence, it's probably more worthwhile to market idols rather than good singers.