It's more a matter of, skirt movement is something they designed a long time ago, and it is common to all kinds of garments. It's not as though skirt movement is random - it's a scripted movement pattern that moves in exactly the same way every time. Use a weaponskill, and your skirt will flare out in the same way each and every time - that pattern of movement was designed to go along with that weaponskill.

The floppy sleeves are a new piece of equipment, and dangle in a way that no previous piece of equipment has. New movement patterns would need to be designed for each and every action you can take in the game. Running, fighting, weaponskilling, even just standing still in the wind. Or, they can just leave them stiff as cardboard. Can you guess which is the easier solution?

I doubt they'll ever fix this. Just take a look at Mi'qote female's default /doze on bed position while wearing a skirt. It's still a tent, after all these years!