Why would my ex try to keep me on his health insurance plan?

My ex and I have been separated for almost 2 years now. We share 2 children that we coparent together so we still see each other on a weekly basis. He recently got a new job and health insurance and asked for me to sign papers to keep me on his health insurance plan but I told him I'd rather just buy my own health insurance since him and I are separated and he was upset and told me I was being irresponsible with my money since that would be more expensive and insists I stay on his. Why is he wanting me to stay on his insurance so bad?
Why would my ex try to keep me on his health insurance plan?
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