I believe it's due to about 600 years of religious and philosophical misunderstandings. Some of these have to do with the internal/external divide Descartes, some have to do with facts vs values and the role of feelings with values (Hume), and some have to do with the smashing together of values and norms by the church and various philosophers from Augustine to Luther. (The greeks and nonwestern societies are not so confused about them.) [See the meaning crisis videos, or Taylor/Dreyfus]

The identification of norms and values is made more complex because they often come bundled together. A person may make a choice because they want to be a gentleman, or a good scientist, and such complex motivations bundle up a mix of norms and values. For example, the person may conceive of a good scientist as someone with deep curiosity and a high citation count. They may conceive of a good man as someone who is kind and proper. And their notions of kindness and properness themselves may be such bundles. Their kindness may contain elements of conformance, intended as a show for others (such as empathic sounds) and other elements of value. If they want to sort out where they are complying with (or setting) norms, and where they are expressing values, they may need to clarify much within their own thoughts.

My assertion, which can only be verified by the reader through their own introspective exercises, is that the process of unbundling described above can often succeed: it can proceed to the point where the norms that make up a bundle are mostly separable from the values, and their separate epidemiologies and developmental circumstances can be traced. As an exercise, I recommend attempting to unbundle a word like masculinity or kindness for yourself, going at least four levels deep.