Having been a VC-backed wannabe for a period of time (and pitched some well known investors), I learned that most investors would rather not offer that kind of feedback because it inevitably raises counterpoints which extends the conversation they didn't want to have in the first place. A simple "no, thank you" let's them get on with their day.

It sucks for the founders but that's really just the nature of being a "salesperson" in any field - constant rejection and little insight.