A lot of it is because they are PERFORMERS and don't just sing. They dance and run all over the stage. I realize the majority of you are fatasses and have never ever exercised a day in your life, but it is extremely hard to exercise and even talk at the same time, let alone sing a song, which these performers would basically be doing.
Next time you're on the treadmill (LOL), even just jogging, put on some headphones and try to sing a long to a song especially one that has some long winded notes like how Mariah Carey sings in some of her songs. You won't make it more than probably 5 seconds before you're out of breathe and have to turn the treadmill off.
Comparing something like the Beatles video up above where they are stationary just standing there playing instruments and singing isn't even comparable to running around a stage while singing.
(and yes there is some exaggeration in my post but the point still stands)