Universities, especially in Europe, base hiring and tenure decisions in part on
where a researcher's papers are published. A paper published on arxiv has less value to advancing one's career than one published in a journal run by a well-known academic publisher.
This also makes it hard for researchers to organize their own journals, separate from the publishers. Many universities (again, particularly in Europe) use the publishing company as a proxy for the quality of a journal, rather than letting researchers within a field make that determination on their own. Thus researchers are forced to stick with the exploitative publishing companies because universities demand it, despite spending most of their time reading and sharing preprint copies of papers.