No kidding a cup holds water, just like a cloud holds water...
True, but "Magic" doesn't see clouds as holding water, because not all clouds rain water down on us. Magic does see a cup as having that purpose and only that purpose, to hold water. So while a cloud *can* hold water (and scientifically, always does), a cup was meant to hold it. When we see a cup, we see a drinking vessel, something that can contain water. When we see a cloud, we usually don't see it as containing water unless it's all dark and stormy.
So even though clouds have water and are associated with air, that's not how we--or the ancient pagans who came up with these symbols--view them.
The stickier issue is that of swords and wands. In some decks, their elements are switched, and the Sword is fire and the Wand is Air. Some people argue that the sword, forged in fire, should be fire. Most argue that the sword, which cuts and slices (like a cold wind), and, in battle, moves swiftly through the air, is Air--also that since Air is associated with communication and talk that it should go with a clanging sword, which is a physical representation of a verbal argument. The clever swordsman, seeing an opening, wins the argument.
Wands wise, the Air people see wands as wood and trees as related to air. The Fire people see wood as being able to burn, as torches and matches. So wands are fire.
It's totally up to you which rings right. But either way, there's no air in wood, no air in swords. There is no fire in them, either. Just the potential to be "on fire" when creating the sword or burning the wood. My point being, questioning why these elements belong to these symbols starts to get really sticky if you get scientific about it
Tarot is poetic and literary and artful, not scientific. So we think about flowing emotions--tears of sadness or joy--and think "water" and Cups are a good emblem for water. We say, "Your words cut me like a knife!" and so associate something that cuts with communication--to communicate we must breathe. So Swords become air. We say, "you have a fiery passion" or a "blazing temper." We associate these with fire; and what usually catches fire? Wood. So Wands become "fire." And we get real literal with the Pents--coin is what you use to buy things. Things are solid, like earth. Pents are earth.
These four emblems and four elements are from ancient paganism. They may not make much sense in the here and now, but they are what they are: symbolic shorthand for different aspects of humankind and the human experience.
Yes? No?