Thing is, do you want to be the SDF (System Defence Fleet, the space version of PDF) fleet captain who pissed off a real Rogue Trader by daring to ask for credentials?
>Piss off someone by doing basic thing that everyone should technically do in space
I mean it's not like they're going to have huge I AM ROGUE TRADER painted on their ship, it's the usual shit to hail a ship and ask "WHO THE FUCK YE ARE". Literally everyone in Imperial space has to have some kind of documents on them to fly around, and while WH40k tech is shit I guess they maybe have some kind of sending pictures of that shit through or something, if the Warrant itself is not in some way similar to Inquisitorial Rosette and has some electronical identification.
The obvious exception are Black Ships but those officially don't even exist and in their case your SDF is going to get blasted just for seeing one so there's that.
These are guys who own industries that span multiple worlds, wipe out some smaller xenos races for getting in their way and duel people to the death for wearing the same outfit as them at a party.
Those are also the guys who go around in ramshackle shitty Sloops and basically live from day to day because they don't own anything except their piece of garbage ship.
If you ask one for his warrant and he turns out to be legit (not that the average SDF mook will be able to tell) there's a good chance you'll be fired or worse.
Or you're just doing your job. And not to mention that it wouldn't proably be the Rogue Trader on the phone anyway but one of his mooks speaking because real Rogue Trader spends his time during regular voyages drinking bitches and fucking wine.
Not to mention the problems inherent in getting into a pissing match with someone who owns a ship(s) the size of a Frigate, Cruiser or possibly even Battleship? Those things can drop lance battery fire onto the house of your commanding officer from orbit. That's generally not a good thing to provoke.
I am pretty sure that fake Rogue Trader wouldn't go around in a Battleship, and if he would do, then HOLY SHIT CALL THE INQUSITION BECAUSE THERE'S CHAOS AT WORK HERE.
Chapter Masters and Inquisitors have the power to determine what Imperial Policy is within in Imperial Space. Rogue Traders do not. They are empowered by but not representative of the Emperor's authority within Imperial Space. They do not have any real authority in Imperial Space, just autonomy. That doesn't make them invincible. And from my reading, Inquisitors do not give a fuck about Warrants of Trade. It is no barrier to investigation or prosecution. A Warrant of Trade isn't even really a defense against accusations of heresy and/or xenos-love if a given Inquisitor decides it's not.
And Rogue Trader families own fleets sometimes bigger and better than your local Navy forces. Sometimes they own huge fucking own trade empires and have lesser Space Marine Chapters at their disposal. Rogue Traders are pretty much equals of any other "powerful" dude in Imperium.
And Inquisitors technically don't give a fuck about anything, if High Lords of Terra were suspected they would go to investigate and prosecute, but the Warrant might still protect you from Inquisitor going "HAERSY" and shooting you on spot if you're caught with some sexy felinid ladies.
Basically, Rogue Traders are given both the power and the rope with which to hang themselves. The smart ones tread that line carefully.
So are Inquisitors. So are Chapter Masters (as hilariously presented in /tg/ legendary game). So...