Years ago, when I was heavily involved in hiring, I asked our CTO whether we can provide feedback to rejected candidates, because it could benefit them. The CTO answered that it may become a legal quagmire if a candidate decides to sue due to perceived discrimination, or something, based on the feedback, even without any merit. The probability is very low but the downside is very bad. So we had to abstain from giving feedback :(
Anyhow, it's not even the feedback the problem, it is that I have enough work experience to understand some of those startups seem to operate on a thin line between what is a technical assignment related directly to their core tech and getting free consultancy. The least they could provide to candidates who have involved time is what was expected.
Sure it is a weak argument, but when you get to cite that possibility and thus save 10 minutes of time creating more detailed feedback (which may or may not be used).
Any feedback you give can potentially be twisted to support some argument of unfair treatment. Even if it's frivolous, employers don't want to spend time dealing with that. So they just say nothing.