Serious relationship and tinder

Hi guys! I would really like your opinion on something. There is this guy I really liked but I found out he has a girlfriend so I didn't try anything. After some time I tried Tinder and I found his profile there. First I thought it was an old profile but then my friend told me she matched with him. Again I thought he broke up with his girlfriend but later I found out he didn't; he's still in a relationship while having an active Tinder profile. Also that relationship is pretty long, I think they are together for over a year. I don't know if he posts anything about her because I don't follow him on social media but I've seen that he has a photo with her as his screensaver. So my question is why are guys doing that? Why be in a long relationship and do all that but still have a Tinder profile?