Recently found out I was the side chick the entire time... Why do men cheat on their long-term girlfriends?

So about two years ago, I began talking to this guy online through one of our mutual friends. We got along immediately and we began to have feelings for each other. I did, at one point, push him away but always went back to giving him a chance because he would say he did not lose feelings for me ever. We started a relationship back in May of last year but I noticed at times he would avoid me or just text me whenever he got the chance, claiming he was always busy (that definitely should've been my first clue) Because of that distance, I broke up with him.

He blocked me from social media and then about a month later, he messages me again saying that he missed me. I still didn't feel like giving another chance because I knew he was leaving for the air force soon, but after his few attempts, saying he still loved me, that I was his first choice, that whatever happened he still wanted me back, that if it became even more serious, that he was to take me away after he got out of technical school, I accepted him back. We went out on one more date, we had intimacy again and then again, went missing and wouldn't talk to me. I started becoming depressed because I could not shake the feeling of being used. He left recently for boot camp and I found out many things, that he was actually in a relationship so I was his side chick the entire time.

Then (I know I did wrong) but after talking to someone who has experienced getting cheated on many times, told me to talk to the girl because I had to put myself in her place and understand how it feels. I did end up talking to her and she told me that they were together for five years. Besides being heartbroken and feeling pathetic for believing every single thing he said and the fact I got used, I was shocked to know he cheated on his girlfriend of many years. I can't understand how a guy can say romantic things to one person while being with another...
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Found out by one of his family members, who was trying to figure out why his brother was doing this, Turns out that I was never the only one. He was with other girls besides me. I just happened to be the one who spoke up.
Recently found out I was the side chick the entire time... Why do men cheat on their long-term girlfriends?
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