Here's why:
* Collective nouns refer to a group of individuals considered as a single unit.
* An audience is a group of people gathered to watch or listen to a performance, presentation, or other event.
While an audience is made up of many individuals, we treat it as a single entity when we talk about it.
For example:
* "The audience applauded enthusiastically." (The whole group applauded, not just one person)
* "The speaker addressed the audience directly." (The speaker talked to the group as a whole)