I’m naturally friendly and I keep in touch with people I’ve worked with. I go out to lunch with them every now and then, etc. I keep in touch with them because I really do like them but a happy side effect is that decades after I last worked with them, they’ll contact me with work.

I know some people can go 30 years and not have any friends they used to work with. At the very least, you should make contact on LinkedIn for your (former) colleagues when you leave a job. Drop them a line once in a while. “Ping” them a text at Christmas, etc. You don’t want to wait until you need work to start building your network.