A couple things-
Before anything else, think of it this way- unlike in "many other card games"
during the battle phase you are not playing a game, you are watching an event. You are drawing balls out of a bag and have no agency 95% of the time so deck order does not matter.
1. There are only 2 cards that do this, clairvoyant and sailor. They are good cards, but they are also heavily contested. You trade a powerful effect for a much lower chance to get duplicates.
2. Challengers isnt magic the gathering. Knowing deck order doesnt matter in 95% of games, and a shuffle consequently doesnt matter because the cards could have been shuffled that way to begin with. It won't make a difference in most games and the difference is marginal at best in the remaining. After you put the card on top with Clairvoyant, whether you shuffle or not the total power of the deck is the same and since you don't know opponent order, and cant influence your own order past the top card, it really doesn't matter.
3. The only time the lack of a shuffle benefits you is when you are running butler/vacuum/sorcerer and so can intricately plan so you never bench out, but these decks already suffer from variance events where you draw your clearers first and bench out quicker than normal regardless. 99% of clairvoyants strength is the card you put on top, and what comes after doesnt matter.
4. This isn't a competitive game, and I'm not the best player ever but I'm current and former #1 arena, former #1 elo, current top 20 elo, and hundreds of games played. Skilled players will slightly edge out others in the long run by consistently making optimal deckbuilding decisions as luck evens out due to law of large numbers, but theres a reason no one has ever achieved 500 elo in this game. Even though this is one of the most played games on the platform and thus has much more elo in the pool to draw from, luck based games like this one keep everyone squashed into the middle with the occasional lucky break shooting players into the 400s, only to quickly fall back down if they keep playing. I recommend you pay some urgent attention to some guides on the forums if you are looking to improve, but shuffle rules arent what makes the game uncompetitive.
I find it very strange that after you look through the whole deck, with cards such as Clairvoyant, the deck does not get shuffled. From my experience in many other card games, if you need to look through the whole deck for cards to then place at the top/bottom/into your hand/etc, you:
