I'm sorry if the question is naive, but if he made such a great amount of personal wealth, why would he still want/need to "get a job" (regardless of company)?
I mean, is that a good way to "leave a legacy"? Why not keep trying on Milk?
There are loads of reasons why someone would rather work for Big Corp. than doing a startup, especially when they've already checked "build a successful company" off their bucket list. I can only speculate, and I don't feel it's right to in what was probably a very personal decision.