I want shared custody of my daughter basically 50 50. But he will not give up primary physical custody why?

The practice of assigning terms to physical custody(like sole, shared, or primary) doesn't really exist anymore, at least in the courts I am familiar with. Child support under the MI Child Support Formula should be determined by assigning both of you equal overnights if you evenly split parenting time. The fathers wish to "primary physical custody" would not have any legal effect. I am also assuming that you are referring to physical custody and not legal custody. Retaining primary or "sole" legal custody would be legally significant.