Why do western gods gain divine power through human beliefs?

Because there are two kinds of gods, one is the god of nature, born with inhumanity and indifference.

One is to believe in God. This kind of god was born in the process of human struggle with nature and natural gods. It was conceived by human beings to fight against natural disasters and evil spirits (another name for natural gods). Human beings created God with the power of faith, and God sheltered and fed back human beings, reaching a mutually beneficial and win-win relationship.

The latter god feeds on faith and has great power. They have needs, so they have a human side. They will protect mankind, develop their beliefs, and fight against natural gods (also known as evil gods) for the growth of their beliefs.