Sun Wukong was transformed from the divine stone used to mend the sky, inherently carrying merit. Plus, being nurtured by the essence of Heaven and Earth for tens of millions of years, he was originally supposed to be born in the form of an Innate Divine Demon.
However, due to a sudden drastic change in the environment of Heaven and Earth, he developed poorly and was born as a normal Innate living being. This is truly like a phoenix devolving into a wild hen; the difference is colossal.
With such a loss of original source, anyone else would be deemed wasted. But Sun Wukong is different. He not only has merit, but also backing. So, he wasn't wasted.
First, a famous teacher laid the foundation for him, followed by someone providing him with Pan Peach and Golden Pill. Finally, a kind old man helped him refine the medicinal powers.
In such a manner, parts of Sun Wukong's original source were replenished. But this still isn't enough, far from enough.
