Is that totally true though? Didn't Frontier First Encounters have some linear series of missions related to articles in the in-game newspapers (starting with the Wiccan Ware Race, followed by shipping medicines to an outbreak system, etc.), which could be considered to be a campaign?
FFE had some custom one-off missions, but most of them were just "things which happened" and there wasn't a lot of connection between them in terms of a big overarching story. You could do none, any or almost all [1] of them as you chose ... and with the exception of the final one were just reskinned courier/passenger/assassination missions.
There was also a big overarching story about the disappearance/return of the Thargoids, but only two of the custom missions were tied to it (including the big final one): mainly it was something you read about in the newspapers.
You can get the same sort of experience in Elite Dangerous by reading Galnet and then doing all the related CGs when they occur. There's also the tutorials [2] and the intro mission chain, I suppose ... plus things like Ram Tah's mission or the Thargoid fight cycle which have custom missions not tied to specific timed events.
[1] Given the timings and locations you'd need a *really* fast ship to do all of them, and the "assassinate this guy" and "assassinate the guy who assassinated that guy" missions were of course mutually exclusive. (If you tried taking the second one having completed the first, then they thanked you for making their investigation a lot easier, and then executed you.)
[2] Given all the complications of trying to run a single-player story in a persistent multiplayer galaxy, the tutorials would probably be the best place to put a Freespace-style campaign. There's bits of one there already - if they extended the tutorials to cover some of the other in-game basics then they could probably do it.