As someone who works at IBM currently: because they have the best technology in my area of expertise. Some of their tech in dying industries is sub-par, sure, but you're hard pressed to find a better security appliance than QRadar. Couple that with the many, many security acquisitions they've made over the last year or so, and it's easy to see why someone would go there and stay there for a few years. It's hard to say no when your coworker is Bruce Schneier, for example.

https://www.schneier.com/blog/archives/2016/02/resilient_sys...