Why do men lie about their aged women?

Long story short I met a guy a few years ago and he told me he was 21 . I was 23 so I was OK with a few years in an age gap. About a year later I check his social media just to see what he’s up to and I caught him on a live stream admitting to being 21 this year. mind you that’s how old he was supposed to be years ago. Why did he lie to me?
come to find out he was 19 and I was 23. And he never told me. we dated for months and I never knew.
Why do men lie about their aged women?
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