Except, well... you talk about only nobles wearing some things. The PCs are likely to be richer than many nobles. Which means they're likely going to wear noble clothing. "Fine clothing" only costs 15 gp, after all.
D&D's economy is ROTFL. But this is known.
Still confusing noble clothing for court (which is typical in artist depictions) and Noble everyday wear.
And it's not like 'adventurers' are wearing their superfine on their dungeon delves.
Secondly, you can't say "no, that's 12th century," or "no, that's 16th century," because D&D doesn't take place in any of those centuries. It takes place in a nebulous time period that spans from the "Dark Ages" to beyond the Renaissance, and contains dozens of different cultural influences--some of which are outside that millennium and some of which are outside Europe, and some of which are completely made up.
D&D always had a dark-ages/medieval tolkienesque aesthetic from day 1.
I do agree however that this aesthetic has become more and more diluted over time and has seemed to really pick up steam post 3e.
So your average player is going to look up medieval clothing, find something like this
Anyone can pic the worst possible picture from a basic internet search.
and think, OK, my options are big dresses (if I'm playing a girl) or a giant blouse, biker shorts or tights, plus jock strap (if I'm playing a guy), and a silly hat (for any gender).
Your codpiece pic is a 17th century painting, and I already covered this in my previous post.
But there are plenty of pictures of medieval art and reenactors where there is nary a Codpiece to be seen.
Or go to more Dark ages depictions with nary a bit of hose or codpieces to be seen as well.
And that's not taking into consideration that nonhuman clothing is likely quite different, and the human PCs could be from a place that's not not!Europe.
So it's completely understandable that players are going to throw that away in favor of something that's not at all "medieval" but looks cool and appropriately fantastic.
D&D art has always drawn its demi humans to dress similarly to the humans.
And I already covered your second sentence here:
even with guidelines of what medieval people actually wore, there is of course room for the artists to stylize things to make them a bit more fantasy. i.e. more "adventurer cool"...
There is a difference between "appropriately fantastic", and straight out of 18th century fashion plates.
The notion that D&D is even vaguely medieval needs to be entirely abandoned, if it has ever been that then it was being ignored a very long time
Which is sad. I feel that the game loses a lot of the heroic Knights vs Dragons flavor it once had by becoming some kind of illogical hodgepodge of styles.
About clothing, it is like Game of Thrones. We are talking abut civilitations and cultures with more a thousands years.
Which makes my point.
GoT has a strong late medieval European aesthetic. With clothing to match.
Of course the clothing has been stylized to make it more "Westeros".
GoT or the series Vikings are good examples of basically period/era correct wear, but given a bit of a "stylized" treatment.
People seem to think I'm advocating for 'period correct of bust!'
I'm not.
All I'm saying is that you can have a 'dark ages-medieval era' correct look and still give it the touches that make it "D&D".
Stuff like Eberron - settings different from the medieval baseline will have different looks, naturally.