Originally Posted by roach
Because Avengers is the name of the team.
When Batman and Superman team up they are not known by the name World's Finest. It's the name of the comic book.
It really should be called World's Finest because that's what batman and Superman are... two of the finest men this world will see. Similar to how the NYPD are referred to as "New York's finest", World's Finest is a perfect title that aptly describes the two core characters.
While I won't claim to know what the film will be called, I do believe that Batman vs Superman is simply a working title, if that. It's a simple way for the filmmakers to refer to this as-yet-untitled film when describing it.