Why would a guy never mention his girlfriend?

I Started a job as a car photographer about 7 weeks ago and work closely with a guy who’s has been such a god send with helping me when I don’t know something to do with a car. We have good vibe etc never really been to flirty but friendly. I show TikTok’s we talk about weekends plans etc. but he has a girlfriend and never mentions her ever. He will say il doing something this weekend but be very vague I don’t understand why.

Why would a guy never mention his girlfriend?
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