Why would a guy keep in touch with his ex to stay as friends?

I’m the ex and I’ve noticed that he looks for me whenever he gets a chance because side he’s a business man. Today he asked me when can he see me again and he is the same always but a little bit more close to me. He keeps saying that whenever I want I can see him or take me to eat. He’s partner did had a cosmetic surgery and her body looks perfect why would he still look for me if I look like his current partner but before surgery. I mean I understand we’re friends and both business owners in different categories.
Why would a guy keep in touch with his ex to stay as friends?
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