At least in the EU, in public universities (most of the top universities are public), publishing papers and getting citations are critical metrics for getting g

At least in the EU, in public universities (most of the top universities are public), publishing papers and getting citations are critical metrics for getting grants and any kind of position. Post-docs often need to get a grant to fund their position in a group, open positions are rare. Usually, it's not the university paying you, you bring in the money from a public funding application, and the university and your supervisor take a cut. These positions tend to last at most 3 years, then you need to do it all over again to get a new position, often in a totally different country, and repeat that for 10-15 years until you are able to get tenure somewhere, or some other kind of semi-permantent position like assistant professor. Then once you have tenure, if you want to advance your career and start a research group, you need to keep getting bigger grants for PhD and Post-doc positions within the group. For all of this getting publication "points" is absolutely critical. The situation in academia right now is horribly competitive and metrics focused, for "fairness". Frankly, society pushed too many people onto studying pure sciences, and we really don't need that many full-time researchers and professors, relatively speaking, with the upmost respect of publicly funded R&D. So it's not hard to (extort) make researchers pay to get their work published, instead of the other way around. Of course, the reputation of the journal or conference counts, but these paper-mills can reduce the risk of not getting published and give you a chance to at least get some citations.