Eh I dunno, salt water is a pretty nasty environment, not to mention fouling, storms, and birds pooping on deck /s. Once the tech level is high enough (and SS is a ridiculously high tech level, just one that has been in decline) then space is sterile in comparison.
And while you can't buy a US navy carrier, you can buy a Russian one, or some of their old subs, and probably some surface ships. Meanwhile private companies develop and build the warship and fighter jets we are familiar with - all it would take is a regulatory change and private people/companies could buy them. In a place as horribly dangerous as the Sector, why wouldn't respectable traders be able to buy military ships to protect themselves?
In the age of sail companies, rich individuals, and developing nations all bought warships and the armaments for them countries with the capability to build them, so its not even historically unprecedented.