Another large client base for paper mills are doctors in countries where English is not a common language, and doctors are ranked based on publications rather t

Another large client base for paper mills are doctors in countries where English is not a common language, and doctors are ranked based on publications rather than clinical work: they don't have time or training in research, their clinical work is in their native language, but promotions being entirely tied to publication count sets up a strong incentive to launder publications. Hospital admins aren't the most savvy about which articles in which journals are respectable, so they check boxes based on impact factor. The doctors write one paper themselves (to give seminars on) and pad their resume with a few papers nobody will ever ask questions about.