Your first point is substantially caused by the issues about sexuality that you highlighted. Sex can reasonably be explained a "special hug" to a two-year-old. But I'm not prepared to give truthful, coherent, useful answers to any followup questions, so I'm ready to say "ask when you're older." I think a young child can understand that their understanding of the world will increase over time. I agree that negative incentives for curiosity is not good child-raising, but that doesn't mean a parent is required to answer every question.

I also think you underestimate the parents' unwillingness to say "I don't know." That answer shows a lack of curiosity. Since curiosity is mostly a virtue, revealing lack of curiosity to a child is drawing back the veil of omniscience and perfection of the parents. Retaining that veil is useful for controlling the child. Many parents lack the capacity to develop a more sophisticated system for controlling their child, so they stick with what works even though it should be obvious that it could stunt their child's intellectual growth.