It was simply oversight during early design phases. And no, voice actors are not paid that much. Even the famous ones. Only Hollywood class voice actors are truly expensive. It's not about the money, it's an inconvenience for the company to hire the whole sound team for like 10 minutes of voice acting.
Oh, and just remove that line from the game already.
Plus, if they followed precedent, they would have to pay for the Audio guy from Relic to be flown to London, presumably business class with hotel expenses etc - hire a studio and engineers - hire the voice actors etc etc
Thread title fixed