Seeing some people talking about things they know nothing or close to nothing about is both funny and frustrating. On the topic of singers (not only kpop idols) singing live there's a whole lot more people don't know about. Let's get methodical about it, shall we?
The diaphragm and the abdominal muscles, no matter how well they are trained, will move if the body is moving creating gaps in the consistency and constant flow of air, thusly leading to pauses in the spoken or sang voice. Since the voice is only produced by the vocal chords that, through friction induced by the air flow, create sound, if the air isn't constant or altogether missing, there will be no sound. As such, if a singer will jump, run and move around more than walking, it will all lead to an imbalanced sound.
It's very normal to see singers using the advantages of lipsyncing and it's quite ok.
An idol, singer, performer is there to perform and put on a good show that will please the audience and bring in money and future events. This means that a show cannot have singers singing off key because they're out of breath (which, again, is a perfectly normal and natural thing when you move and talk or sing at the same time) thus leading to a displeased audience.
An idol is just that, an ideal image. And to keep that image, sacrifices and compromises need to be made.
Now, as a musician with over 30 years of experience and constant training, I can assure anyone that I'd rather go see a performance where idols lipsync but dance flawlessly rather than see them suffer all the while torturing my ears in their attempts to sing live.
On a different note, having to go through a live show or even to record a performance is very demanding for the body and a lot of that fatigue will first be noticed on the throat and voice. So, imagine, if you will, having to have daily performances and having to sing live in all of them. No matter how well one is trained, that will ruin a person, the needed time to recuperate being longer depending on how big the damages are.
Also, the music industry, be it Korean, American or whatever you wish, won't take the chances of having displeased audiences. Too much money are involved and too many people's jobs are at stake. So, don't be all so righteous about who sings live and who lipsyncs and just enjoy the show.
