Game prices are increasing anyways so that threat is as empty as it can get. If they don't get you straight up, it will be through microtransactions, DLC, or whatever else they can cook up.
While I agree with it in principle, I certainly don't think they should be entitled to residuals and royalties before the people that actually spent years of their lives making the game.
While they contribute, no one buys a game for its voice casting.
Additionally, some of these actors can get paid six figures for a few days of work, far more than anyone that actually developed the game.
For example, I think it was True Crime 2... they paid Lawrence Fishburn like $1M to record for a day, and it was never used. The team crunched for like two years and Activision pushed it out in a terrible state, so they got laid off for all their efforts.
Alright, I'm not in the game industry at all, but to me it seems that the question isn't "why should they" but instead "why don't I?" The current developer-publisher climate didn't happen out of nowhere. But that's an entirely different battle, on an entirely different battlefield--and one that devs will have to fight on their own--because right now it's the VA's turn.
As to why people buy games... they don't buy it for any one piece of content, but the collective effort of everyone--voice actors included. Metal Gear Solid V, for example, lost David Hayter as Snake which is something everyone playing has noticed, but it was still the collective effort of Konami and thus a game that people bought; David's absence wasn't dealbreaking because people wanted to play the game, but it was nonetheless a mournful loss that impacted the game negatively. So VA's matter, but are not a deciding factor, just like every other role in a game's development.
If things are to change, everyone needs to stick together.
Edit: Also, I'm pretty sure big-name Hollywood actors are in their own league, far, far above average video game voice actors. Even the popular ones.