Why would a man stay married in this case?

I've been dating this man for a few years now. Long story short, he told me that he was single and divorced. He said that his ex wife was a control freak and that he was so relieved to be away from her because life with her was miserable. Fast forward, we moved in together, have babies, etc. While cleaning our bedroom, I came across paperwork and couldn't believe it. He's still freakin' married! It was for a legal agreement signed by my boyfriend and his apparent wife, stating that they were legally separated and no longer residing together but had no intentions of ever divorcing and that both parties agree that they don't want to remarry anyone else. I asked him and he said they're not together anymore and that he's with me, so why should I care. I find it annoying because when he had a major medical event happen and was hospitalized, I wasn't allowed to know anything or visit him. The doctor said only his wife and family. They never had children together. She's old, overweight, and not physically attractive at all. He was born in America so it's not a green card situation. They haven't been married a long time. Half the time they've been married, he has been dating me. I don't get it. Why stay married to someone that you're no longer with and claim was a control freak who made you miserable?
Why would a man stay married in this case?
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