I think you missed the actual points people were making.
@prabe mentioned the idea of fate not as a specific explanation, but simply that such ideas will suit some settings more than others, and in such settings could therefore help explain coincidences and the like.
@prabe then mentioned the USPS to point out how, although we in the real world don't often actually personally know the people involved, we know the process and ways to engage with them to get things done. The idea here is that the criminal character knows about things like thieves' cant and graffiti or similar customs that can put him in touch with people who can deliver messages. The character knows how the underworld works and can use that knowledge. He doesn't need to always know someone specifically.
And here's the main thing... there doesn't need to be one explanation to explain how the character is well-connected. In one city... maybe one near the character's home town... he might know someone personally. But a city that's further away? Maybe his cousin once worked with a guy there, and they were close, so the character knows to seek that person out. In another city, maybe he knows how to signal that he's a freelancer in need of some local help, and then is contacted by a local guildmember. There can be all kinds of reasons.
There doesn't need to be one single explanation. As I said, approach the idea with some sense of creativity... look at the relevant factors in each case and come up with something that makes sense.
I mean, if you put zero effort into this, then of course you'll be dissatisfied with the results.