Common advice to students writing papers is to always cite their sources, especially quotes. Why can't students compose research papers entirely out of quotes, properly noted and cited? Why is that considered plagiarism?
Because even with attribution a student work compose entirely of quotes from other people is still fundamentally claiming someone else's words as their own. Students are assigned research papers and projects with the expectation that they will search for material, read it, come to some kind of conclusions about the information, and then be able to present their conclusions in paper or project form. Word for word quotes are supposed to give weight and support to the student's ideas and conclusions, not to replace them entirely. A collection of verbatim quotes from other authors does not demonstrate the student's understanding of the material, nor does it show that the student is able to articulate their conclusions.