Why do coworkers suddenly ignore you when you leave a job?

I mean they act all nice, cool and love having you around and have chats but when you leave a job and it’s your last day, they suddenly ignore you. Some of the coworkers at my former job that liked having me around didn’t seem to care that it was my last day. I wanted to sent them lovely cards and gifts but after the way they treated me, I didn’t give it to them. I was glad to leave because most were toxic but I cried when the ones that I though liked me just ignored me. They didn’t care that I left.
My neighbor told me, “Welp you just learned a hard lesson. Not everyone is your friend and there’s a lot of fake nice people and quite frankly they didn’t really like having you around.” He didn’t seem to care that I was crying and was heartbroken. I told him I never said they were my friends. Then he told me, “Well if they knew, then they should know better. Move on. No use of crying over that shit.”
Every time I get emotional and cry, some people get insensitive and give me a harsh life lesson lecture.