is because they can't write a melody or lyrics.
I mean, these coaches have great technique and all that. But a lot of them (I'm talking the rock-oriented ones) write the cheesiest lyrics and melodies.
For example, take a look at Robert Lunte's performance on his original:
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Robert Lunte always has good advice on his forum posts, and if I had the time and cash I'd go for one his power seminars, but man, this song is just bad.
Bottom-line, however, is a career in music (even in coaching) beats working at the local steel mill.