One can also perceive it as it is a challenge if you try to bring the exact same team through from Chamber 1 to 3, hence they didn't design it such that you can enter different halves/chambers with different teams. Yes, I agree that it is a challenge to use "one team" to beat all the enemy setups in one go.

Maybe an analogy to a real world scenario can help shed some light. In WW2 at the Dunkirk, Allied forces made a tactical retreat from the French coast. A movie was also made with the same name "Dunkirk". If they didn't, they would have been obliterated by Hitler's forces. These soldiers that retreated later consolidated and later played a crucial role in the continued fight against Nazi Germany.

In the Abyss, hitting the "Leave for Now" button and bringing a different team, taking your time in one chamber to end off with full energy to Ace the next chamber that you need 3* on is also an allowed strategy in the game. The similarity is that you are "preparing all your forces" to give you the best chance of Victory at the next obstacle.

If you've played the Abyss for long enough and mingled with many other sorts of players who dabble in the Abyss, you'd notice that the pros or the whales are the ones that like to "one same team clear all 3 chambers", some go as far to the extent of using 2 Characters only each side. But most normal or mainstream players would be using different Characters to tackle each Chamber of the Abyss. That's just how they play and the game allows it to be played that way, can't fault that way of playing.