Ivy, it's simply why they're doing this.
(1) $$$
(2) Hey if Marvel made a fortune from doing cross-overs with their characters, why not us too?
Really I thought you folks who ate up MAN OF STEEL would be all for this?
Honestly I'm more interested in the other blockbuster cross-over superhero movie in summer 2015: AVENGERS 2. In fact I would be shocked if AVENGERS 2 isn't better than [whatever title used for Batman/Superman.]
Speaking of which, Ultron was announced as the villain for that film. The movie will be titled THE AVENGERS: AGE OF ULTRON.