Because for many studios, the creative staff are cogs to be used for a game then dumped when it's over. Read "press reset" for many examples of this. They want

Because for many studios, the creative staff are cogs to be used for a game then dumped when it's over. Read "press reset" for many examples of this. They want you to stick with the publisher for sequels, not follow the developers as they go from studio to studio. There are exceptions to this (Kojima for one).