It would be hard for investors to give as much feedback as an American Idol judge because they have so much less information. AI judges have actually seen the person perform. An investor merely talks to them about what they would do.
Often you have no more than a gut feel. E.g. "This person seems kind of ineffectual." It would be useless as well as insulting to be candid in that case.
Transparency is good for the entrepeneurs, but perhaps not for VCs.
It must take a lot of work to prepare an insightful honest answer. A shady answer gets the entrepeneur out the door without the need for putting together a well-structured argument.
I agree that it takes more work, it also takes more work for entrepreneurs to be transparent, honest and open. It's always easier to hide. At least it will always feel easier and safer.
VC's almost unanimously claim that they like what they do because they get to be involved in helping entrepreneurs succeed. I don't doubt that to be true. They could be more helpful with a bit of effort put into providing true, honest feedback to entrepreneurs.
Then again, unlike judging singing ability, the chances of being correct as often as Simon is are low.
Which probably explains why there are so many Randys among the VC crowd.