Corporations only behave as logically as the people running them. Maybe the individuals making the decisions don't feel like turnover is a problem they can feasibly solve, so they just go along with it. Maybe the problems regarding turnover are hard to communicate up to higher management levels, so they rarely implement strategies to reduce it. Maybe most companies value making workers feel expendable more than they value low turnover, regardless of whether this is a sound (or even conscious) decision on the part of upper management.