Because most journal articles are speaking to multiple audiences—those wanting to efficiently consume the text in a literature review (like the instruction in this article) and others who are preparing a rebuttal or an elaboration on the methods; still others are trying to understand the larger discourse / set of questions in which the paper is embedded, and don't particularly care about the specific results of any one. Journal articles as they are currently written are a decent compromise among the needs for all of these populations.