It's a shame too, because I'm sure there's a lot of startups that could have used some honesty from the people around them early on to modify and pivot and consequently not fail.
Challenging an entrepreneur on the validity of an idea is actually a form of nice feedback. It seems like it's saying, "I believe in you but I have found some concrete roadblock that will cause this to fail." That's almost always bullshit, because startups change forms throughout their life, shifting strategies and tactics to avoid such roadblocks.
Really, that feedback is saying "I don't think you have what it takes to get around roadblocks."
Guess what happened?
They became defensive, trying to explain to me why my feedback was wrong. One person stopped discussing their project with me entirely, and some others just avoided me after that.
Why go through all that when you can just tell them their baby is beautiful, give them some token softball criticism that they can say they're already addressing and move on with your life?