Swords, feudalism, badassery, and no-one knowing what the forget is going on, all while suited in full plate.
The dark ages were a magical time.
plate armor, swords, and the dark ages are three things of different time periods. early western plate armor was the roman lorica segmentata and that stupid hellenic muscle cuirass. it was expensive as stuff to produce and maintain, so it wasn't really given to people who weren't rich or high brass. the era of rome is not within the dark ages, as the dark ages are generally considered to have started aroouund when the vandals made their name into a word by sacking roma. during the era of rome, though, swords were not
the weapon that armies would charge to battle with. spears were. what good will a sword do to this?

note: spear sizes here are accurate. the phalanx's lack of discipline and armor are not accurate (though those stupid loving helmets are real)
when rome inevitably fell due to the fact that it was becoming a super capitalist society without really understanding economics, leading to hyperinflation, leading to the inability to pay for troops to enforce the government's rule, plate armor sorta faded into obscurity. truth of the matter was, despite what videogames and game of thrones might display, a thick gambeson is actually very good armor. you will not be able to simply slash through a decent gambeson. anyways feudalism emerged from the ashes because byzantium (east rome) managed to not be as handicapped as west rome, who got BTFO'd when the germans invaded and italy became known by the name of the dominant germanic tribe ruling it (lombards; lombardy).
swords never really took hold as THE weapon. it was always pretty much just a dueling weapon or a sidearm, unless you're in the holy roman empire in which case go buy yourself a war knife (kriegsmesser) in order to circumvent the imperial laws saying you can't own swords but can own knives. eventually, economies and metallurgy rose up and true plate armor became viable. but we weren't in the dark ages anymore. by now, the renaissance was just about to begin, as the last crusade had just ended and the crusaders returned with tons of loot from orthodox/byzantine-cultured land.
but yeah, a knight in shining armor wouldn't be charging in with his sword (though he would probably have a sword on his belt, yes). he would be charging in with a mace.

(note: the mace's head would be smaller. also, the chainmail coif he's wearing is also historically inaccurate but that's not relevant to plate armor, swords, or feudalism)
to summarize:
plate armor is everything except dark ages era, and swords were generally never your go-to weapon (OH WAIT I FORGOT TO MENTION THAT THEY WERE IF YOU LIVED IN A stuffHOLE WHERE EVEN GAMBESONS WERE IMPRACTICAL LIKE THE HOT stuffHOLE THAT WAS ANCIENT EGYPT YEAH THE EGYPTIANS HAD USED SWORDS BUT STILL SPEARS WERE PREVALENT TOO)