Why do separate companies develop and publish games?

In very brief: (note, your question is obviously asking about major publishers like EA, with whom I have not personally worked). Publishers generally help secure financing for the long up-front effort on a big title, and as such, tend to choose titles that are in some way similar to their core business. read more

As to why do games from the same publisher look similar, it is because publishers want to minimize their risk of fronting the money for development, marketing and inventory, so they publish the types of games they think will sell well rather than taking the risk of publishing something different. read more

A publisher have 2 games, they both have good average rating and they want to publish another game, which have less resources and effort. Publisher predicted it won't have as good rating as its previous games, but decided to publish it anyway because it needs money. read more

The production of games is a collaborative affair and if you buy a game in a box at retail then invariably it will have at least two company names on it, often more. There will be a publisher and a developer. Often there will be separate publishers for separate territories and sometimes games are developed by multiple developers. read more

Often there will be separate publishers for separate territories and sometimes games are developed by multiple developers. It is common for ports on separate platforms to be handled by a different company and occasionally the single player and multiplayer portions of a game will be made by separate developers. read more