Why does an author use active voice?

An author uses active voice to make his writing clearer and easier to read. Active voice makes for a simpler work. You are in the moment ... you are current ... you are participating in the current stream of events.

Active voice puts the reader in the action. The reader can imagine things happening right now, in the moment. Active voice also gives the sentence a clear actor (you know exactly who/what is doing something), so there is no ambiguity.

An author uses active voice to make his/her writing stronger and more authoritative. Passive voice can make prose more lyrical, but also more confusing.