Why does it seem whenever people get more friends they ditch or forget about the one that introduced them to the new friends?

Like this has happened a lot. I’ll meet a guy at work or school or whatever and he’s got no friends so I’ll introduce him to my friends and other people. Then the second they start talking then he completely stops talking to me. Like I don’t ditch or forget about anyone literally never it’s a blessing and a curse tbh lol
Why does it seem whenever people get more friends they ditch or forget about the one that introduced them to the new friends?
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