I think the reasons depend on the company, too. Large companies have different reasons than indie devs and amateurs.
As an amateur game programmer, I have a -ton- of ideas. I don't need anyone else's because I've got too many of my own.
As an indie dev, I imagine that the above applies, plus they already have a game or 2 released in a certain genre, and changing genres can be problematic with the fans.
As a major developer, you can break the 1-genre rule, but you become a target for lawsuits, as jonnathanson noted in another comment here.
As an amateur game programmer, I have a -ton- of ideas. I don't need anyone else's because I've got too many of my own.
As an indie dev, I imagine that the above applies, plus they already have a game or 2 released in a certain genre, and changing genres can be problematic with the fans.
As a major developer, you can break the 1-genre rule, but you become a target for lawsuits, as jonnathanson noted in another comment here.