Why do guys lie and say they're single?

So I got speaking to this guy, said he was single, took my number.. Asked me out blah blah blah. All was good but there was something in the back of my head telling me not to trust this guy. So I dedided to have a little stalk on his Instagram and came across his girlfriends page... Oh it gets better he then begged me not to say anything to her even though he had lied to both of us and I of course told her because I'm an actually decent fucking human being and no person deserves to be treated like a piece of shit... But please tell me.. Why do men lie about their relationships?
Why do guys lie and say they're single?
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