Because all power shall be harnessed by the physique, depending on the strength of the body as its foundation.
However,
Blood Exchange is a process.
Once the life-and-death barrier is crossed and the preliminary Blood Exchange is completed, the process of Heaven and Earth Infusion will cease, but the Blood Exchange won't be finished at this point. In the subsequent period, a Martial Artist still needs to actively absorb the Power of Heaven and Earth to refine Martial Blood, making the entire Martial Blood more refined until it reaches the point of no further advancement, which is the end of Blood Exchange.
This process is much faster than the cultivation of Marrow Cleansing Realm, because the Marrow Cleansing process also refines the entire Martial Body, but when reaching the Blood Exchange Realm, the refinement of the Martial Body is completely finished, only needing to refine the Martial Blood.
