It's a little more nuanced than that, I think.
Let's go with the hypothetical scenario that Bayonetta made 450 Million USD, right? If we take certain factors into account (Region pricing, discount on sales, etc) and the fact that there are only two games in the franchise, that would put the sales somewhere between 10 million and 15 million copies. For reference: That is Halo 1 - Halo 2 levels of success we're talking about.
That is a huge audience for a franchise, and when your audience is that big, you can't rule out that the voice acting can be important to a certain part of your audience. This also opens up all sorts of other opportunities: Spin offs, merchandising. Licensing. It makes sense to invest in the face of the game, which is the main character and voice acting is an essential characteristic. To use an extreme example: People don't purchase a Mario game for its voice acting, but you can bet that Martinet is paid millions for his performances, even when they are often little more than yelps.