You probably just never got to know them that well and did things outside of work with them. I've met people at the office and stayed in touch with them and hung out after they left or I left the company.
That's a great point I never thought of! I'm also an introvert and don't relate to my peers who normally are always older than me by 10 years for some reason lol
That does make it tough at times when there's the age gap. Every younger place I've been at has been lord of the flies so I've since then looked for places where people are a bit older lol. One of my good friends now is about 10 years older and I've found once you get past 30 whether you click with them matters more. One piece of advice I'd say is just try and find people where you do things with them regularly outside of work.