Guys, my friend was hiding his girlfriend from me. Why?

I had a guy "friend" since less than a year that I liked a lot. He gave me too much attention at times (wanting to know trivial details of my life i. e. if I had a good night's rest, what classes I have that day etc., and he shared at the same level). It was always me he contacted when he was stressed or something important happened in his life, reaching out to me to let me know of stuff such as if he caught his flight or how his exam went. Strangely he always avoided meeting outside and when I confronted him he gave me weird excuses such as I don't have time. Recently I found out that he has a girlfriend and was shocked, especially because he became angry and defensive about it. I was always confused by his behaviour and couldn't figure out if he liked me more than friends given how we texted everyday and shared stuff that a person would normally share with a girlfriend or boyfriend and yet, never wanted to meet outside and never tried to make things physical. I have been thinking he was hiding it on purpose not to lose me because with close friends, this topic comes up naturally in daily conversations. I assume he respected and cared about me at least a little because he never made things physical and always wanted to support me but still feel led on emotionally. What do you think? Why did he hide it?
Guys, my friend was hiding his girlfriend from me. Why?
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