Who says the noble thingy really DOES something
The empire survived the Barracks emperors because the Bwaps kept running the show. So who says that the nobles are more than empty figureheads doing 3-4 "presentations" a year. Sure, they command "power" but who is keeping them from delegating them to a Senneshall and just "attend important social functions (Party, Drugs, Groupies), "present high nobility to the loyal citizens" (Travell around a lot) and not to forget "promote the sciences" (Hang around archeological digs)
Don't forget one of the maind differences between the Civil War of 600 and the Rebelion is just that.
Civil war ws a fight among several High Military ranks (mostly, if not all, Navy admirals), but the bureaucracy keep the empire runing, ignoring the war when it could, and bowing (or pretending so) to the closest admiral when needed. For what I've readed about it, it seems that Imperial Nobility was part of the latter, not the former.
There are no High Nobility named as taking sides in the Civil War. IMO, for what I know as how the Imperium works, High Nobility are the heads of Imperial bureaucracy, and as such, they didn't take places, and that was what allowed the Imperium run, even in the Navy quarreled among themselves.
Who says the nobles are "well known"
I would not recognize our current EU president (don't even know it's name, don't care for the airwastes in Brussel) and it is a lot closer and with less "travel time" for the news than an archduke. Add in that the 3I does not rule the worlds and chances are any nobel thingy above "local baron" (The parasite that sucks of our hard earned money) is off little to no interest to most planetary citizens. Similar to the current day EU and most citizens. So a higher noble can run around without being recognized most of the time
Neither I know who's the EU president right now, but, after all, he has quite less power than any of the 'inferior' presidents of the states members. The UE is (IMO) a bad example, as most states try to mantain their own presence and power at its expense, and so try to have the lowest profile possible for its teoretically higher level of representation.
As I told above, I see the local hight nobility as the heads of local Imperial bureaucracy, and as such, probably more known persons.
And even if I'm not right on this, through you and me don't know who's now the EU president, do you really belive he (I'm nearly sure is not she) can travel easly incognito? He'll sure be recognized by many people who's more interested, followed by some press, and sure he must have at least some entourage of advisors and seccurity people, even if just to mantain the facade that he makes something and the money put on mantainig his post is not as wasted as most people thinks.