Why do you call it the passive voice?

Answer 1

The passive voice is used to describe an action of some kind acted onto the subject. While this is useful for drawing attention to things being acted upon, overuse of it creates a stale tone, like one might see in legal documents. Often, the attention should be drawn to that which does the action, providing a more vibrant tone.

Answer 2

It is called passive because it is the opposite of active. In active sentences we can see who or what does the action of the verb:

The cat chased the mouse. -- The cat does the action = chase. This an active sentence.

The mouse was chased. -- We don't know who or what chased the mouse. This is a passive sentence.

If you want to say who or what does the action in a passive sentence then you use:

by + noun phrase

The mouse was chased by the cat.

Passive is used when the actor/doer of the action is not important.

Example: The Mona Lisa was painted in the 16th century.

Passive can be used when describing a process, where it doesn't matter who does the action.

Example: Tea is grown on plantations. It is pickedand dried, and then it is packed into boxes.