Perhaps IBM figured that they don't need that many chips to warrant its own fab, which are expensive to maintain/upgrade to keep up with cutting edge process technologies.
I can believe that it made more financial sense for someone else to actually make the chips it wants (like nVidia, Apple and almost everyone else besides Intel and Samsung) than running its own fab in the long run due to economies of scale involved in fabs.
I can believe that it made more financial sense for someone else to actually make the chips it wants (like nVidia, Apple and almost everyone else besides Intel and Samsung) than running its own fab in the long run due to economies of scale involved in fabs.