That's not a fact at all. Astarion's suit is nothing like the exampled pictures, and is clearly supposed to be a doublet, which in case you're unaware was a kind of battle jacket. Wyll's frock is visibly padded. And Korean games are completely irrelevant.,
You are free to wear just clothing. Both Barbarians and Monks are designed to wear nothing but clothing. But DnD do not give clothing AC, and neither should BG3. Mages don't wear armour either (unless they invest in feats or get proficiencies from other sources), they wear clothing.