I don't think you realize how much work goes into perfecting a voice, not just with singing live music, but also dancing, "putting on a show", being powerful, etc. . . With a guitar, the strings are on the neck of a guitar, with singing the strings are in the neck of a human being. . . It's just different.
I am a singer, and I will have bad days where no matter how much tea I drink, no matter how many warmups I do, the strength in my voice just isn't there. And then the next day I can hit the highest notes of my range like I didn't just have a day where I could barely sing at all.
A lot of the time that you hear radio songs, the vocals aren't sang in one take (or even two or three), it's like a hundred takes, over several weeks of recording, for one verse. . . Some singers have put in thousands of hours of work and training, just to go in and get 100 takes of the same little tiny run or harmony that will play for 3 seconds in the middle of the chorus.... to perfect the song... and they would STILL need to put in serious effort to put on a grandiose live performance.
People go to the concert for the music, and the thousands of people there listening and dancing, the atmosphere and feeling of the crowds rawr, and the SLIGHT chance to meet their favorite celebrities; not specifically to hear the singers voice. . . I'm not paying $700 so that I can hear Miley Cyrus's voice specifically, I am paying $700 because I want to rock out and cry as I sing The Climb with a thousand like minded people and the coolest rock star to ever live, who lives a double life as Hannah Montana.
Plus, often times it's not "full" lip syncing, it's just backing tracks so the singer can take a breath without the audio being jarring, it's so they can dance and put on a show without losing too much of the song. A lot of the time, they are still singing somewhat.