Why do exes come back?

My exe of 6 years left me for someone else. I didn't try to gain any type of understanding I just moved on. 5 months later I am in a new relationship however my exe still calls me every week or month. Either he is calling cussing me out saying things like if it doesn't work out with my man don't call him lol. Or he's trying to have sex or hang out. He stalks me on Instagram etc. Why? He is still with the woman he left me for so why not be with her fully?
Why do exes come back?
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