Ask HN: Why do companies reject you without giving a reason?

I just got rejected by a startup for a vague reason that I was not the right fit for the position they were looking for.

I am thinking of building a app for people to list such companies to warn other applicants, in process defaming such companies. Rejecting a candidate without a reason is absolutely rude cause it leaves you confused on how to improve your self. And companies who conduct themselves in such manner probably have a devious reason for rejecting you. (ex: Racism, gender bias, heightism and what not?)

You think this app is a good idea?