Why? The VP has little real power. It's not a demanding job, essentially is an ambassador with the power to break a tie vote in the Senate. She hates Trump and would do whatever it takes to stop him, which includes accepting the VP job.
Michelle is agenda minded and would like nothing more than to see her husband back in the mix. Michelle also knows she would not win the Democratic Nomination. The process of becoming the President is a real grind and they like old and white based on their final results. Many people would say yes to the job if the path was easy.
My question is why wouldn't she be Biden's running mate if he asked?
Doesn’t matter who it is. I wouldn’t be shocked if he picked Kermit the Frog.