Why do girls blame other girls or call another girl a liar when confronted about a cheating boyfriend?

So my boyfriend cheated, probably whole relationship but I saw his ex pop back up on social media in March, I accused him of cheating end of April, he denied said he's not a cheater blah blah. So weeks go by of me crying, not eating, missing work apologizing him saying he didn't know but is see him on occasion stay over but didn't want to talk about anything. May 21st his birthday weekend I saw him and we had sex the Friday, Saturday and Sunday that weekend despite finding another car there Saturday night, caught it again Sunday night so finally Monday I said I deserve better told him what I knew and he's like other women huh, why is this happening, 3 days later that chick from the past no longer single status. I confront him about it all says I'm way off base, they just started talking. He goes away memorial day weekend see her in a picture which he took down asap. I just said thanks. I messaged the girl on Instagram just saying hey I had sex with him those days I assume it's your yellow car, weeks go by nothing. He starts messaging me again saying how I walked away from us, what do I want, we hung out a few times had sex. Girl finally reads messages asking me to send her the texts u did screenshotted stuff, described his house it proves nothing were happy blah blah. Like dumb b doesn't realize I'm not trying to prove he's cheating on her all I wanted to know how long he was cheating on me, I accused him April 29th guaranteed he was laying the groundwork long before that I just wanted to prove he cheated on me but instead she accuses me of being one of two other chicks like wow what am I dealing with saying me and my daughter were with him his birthday weekend like he's sick he had me come over right after they left, but girl keep your man why couldn't girl just give me info I want she can keep the cheat I'm sorry I have him half a chance when he texted me out of no where after 10 days saying things we need to talk about blah blah
Why do girls blame other girls or call another girl a liar when confronted about a cheating boyfriend?
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