You aren’t missing anything. Normally, you wouldn’t need to talk to anyone in that situation. If you were departing from a towered airport under Class B, they would just tell you “frequency change approved” once you were no longer in their airspace. At that point, you wouldn’t need to be talking to anyone, provided you remained clear of the Class B boundaries.
Default ATC is so bad that IMO, it isn’t worth using. It would be a large undertaking to get it right, so I don’t expect Asobo to ever make it work to a satisfactory level. There are some third party options, such as Pilot2ATC and PF3, but they aren’t perfect either.
Really, we need Asobo to further develop the SDK to support a high quality ATC add-on. Specifically, we need an in-sim weather API, as well as either public documentation of the BGL format or (ideally) an API to expose navigation data and airport layout data. Finally, there may be improvements needed to SimConnect for AI aircraft control as well as better AI aircraft behavior in general (i.e. aircraft taking the first available taxiway to exit the runway upon landing, etc.). If we had these things, I have no doubt that great ATC add-ons would be developed by third parties.