Why didn't my ex tell me he had a new girlfriend? We're still friends, I told him about my new guy.

Me & my ex were together for a LONG time. We've been apart almost 2 yrs...but we've never stopped talking. I was open with him about the new guy I started seeing awhile back, however he never told me he was dating the girl I assumed he was..I found out through a friend & he admitted they were together. I understand he doesn't HAVE to tell me anything. But why hide it if I was open with him?
Why didn't my ex tell me he had a new girlfriend? We're still friends, I told him about my new guy.
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