Self citation and plagiarism
Well, the pedantic legal answer to the question of why an academic re-using blocks of text they themselves have written in a previous publication would be plagiarism is that said academic has (almost certainly) assigned the full copyright of the work to the previous journal publisher. Given this is a forced part of academic publishing I would argue that the above is ethically something of a grey area, but legally chances are our hypothetical author no longer owns the text they want to reproduce.
However, I would say you are missing something here! The most efficient thing to do in the case you describe would be to cite the publication containing the data set and original description. Perhaps providing a brief summary in the new paper would be appropriate depending on the specifics of what you're writing about.
A similar example of the above from my own research was experimental equipment descriptions. I realized fairly swiftly that I was often reproducing text describing a particular spectroscopy setup. This can easily be condensed down to: &quot;Technique X was performed as previously described.[REF]&quot; and those who are interested in the exact details can read the referenced paper.