Qin Feng used the majority of the Dragon Spirit Fruit's power on his physical body mainly to prepare for merging with the Rock Giant's Demon Core.
The Rock Giant is an entity capable of contending with a ninth-tier superior Demon Beast, powerful in strength. If Qin Feng's physical body wasn't strong enough, let alone transforming into a Rock Giant, he would find it difficult even to accommodate the Demon Core. Forcing a fusion would only result in damage to his body.
Cultivators seek the Dao for longevity, and if one damages the Taoist Body just to obtain some strength, it would be counterproductive.
However, after refining several Dragon Spirit Fruits, his Azure Dragon Daoist Body's resilience had become barely sufficient to withstand the transformation into a Rock Giant.
Yet Qin Feng did not immediately merge with the Rock Giant's Demon Core. Firstly, he hadn't completely erased the Rock Giant's soul's rebellious will, and a rash fusion could lead to backlash.
