A lot of talk here that colleagues aren't friends. What is wrong with you people? If you like your colleagues you can invite them to do stuff after you or they leave. They probably haven't invited you because they never got around to it either. Most of the older generation people met through work is the only way to make friends. No wonder loneliness is so rampant now.
I tend to make few friends. Of those, I made most of my closest friends at school and the Uni, I'm still in touch with them, and we keep our friendships going through good and bad times. It's an age thing, I suppose. If I were still in my 20s I'd probably make friends at work, too.
I tend to occasionally keep in touch with former coworkers, but I'm an introvert, I tend not to enjoy "afterhours" activities, and eventually all these new bonds tend to fade.