LiquidOxygen80: How do you figure? It takes time to get a new programmer up to speed so he doesn't go in blind, destroying potentially months of hard work.
Yes, it takes time to get a programmer up to speed (but not necessarily as long as you seem to think, I've seen programmers do their job normally after a week of getting accustomed to the project, it's particularly easy if the lead makes sure that certain standards which can be easily communicated are kept throughout a game's code), but a voice is something you might not be able to fully replace at all and especially once a character or entire game is tied to a certain recognizable name replacing that person is a problem. And the thing is that actors and their performances are super recognizable, that's why they are also highly useful in marketing and that's also why you absolutely should stick with them once their involvement in your game or franchise is known to the public. Replace them and you risk to disappoint or even piss off fans, news of conflicts and other problems reach the press etc.. It can lead to a serious shitstorm and already has a bunch of times.
Also note that once a project is already going on you can make personnel changes in any department (even if in certain phases of a project it may be problematic), a competent worker will be able to continue the work of his predecessor / fit in with the other members of the department but once voice recordings have been made they become perfectly useless if you replace the actor.
LiquidOxygen80: Also, changing a designer could completely change a game's aesthetics, forcing delays as people are then forced to work with entirely new assets.
Designers are the people responsible for the game design, level design etc. and have usually little to no impact on a game's aesthetics. And no, changes in the art department don't result in the stuff you're describing either. The vast majority of artists in the game industry are craftsmen rather than artists, they just get the job done following the general style specified by the guys in charge rather than expressing themselves creatively. And even if the lead artist got replaced or something, if the new guy as much as suggested redoing a notable amount of assets he wouldn't be doing his job properly and would deserve to get his ass fired (UNLESS the company could actually easily afford to redo all the content in question and much better sales could indeed be expected but that's simply not the case usually).
LiquidOxygen80: A human voice is the most common element in the world, and yes, even David Hayter's terrible Batman impression is replaceable.
Having worked as an audio professional with voice actors and singers (and frequently using my own voice in sound design) I assure you that the human voice is one of the most complex things imaginable and each voice is unique, especially in case of voice acting where you have to take other factors that constitute a performance into account. Sure, you can choose almost anyone for a role but once an actor's performance has already been captured or even his name is tied to a certain character replacing him is simply not an option in many cases. Especially using several actors for the same role in the same game is something that can't be pulled off sensibly.