They tried the divorce right and the judge wouldn't allow it.
If Michael divorced her, he wouldn't get the money.
If he let the parents have her, the money, whatever's left of it, would go with Terri. Plus, he would have to give an accounting of how the money has been spent over the last decade and I have a feeling he REALLY doesn't want to do that.
If he divorces, then he loses any life insurance he would get when she dies. I hope the insurance company fights paying off any claim, on the basis that he killed her.
Michael has been offered millions to divorce her. He wants/needs her to be dead. Wonder why?