The problem is music/voice unions and such have little to no bargaining power in the game industry, because music/voice (performance audio, really) are the only unionized disciplines that appear in game development and production, and even so, there are plenty of (extremely talented) non-union artists and performers. They can't reach to any fellow unions to put pressure, because... well, there are none.
Just look at the AFM fiasco with Austin Wintory. AFM didn't get what they wanted, so they made their own union members suffer for it. The game industry didn't like the terms that AFM had put up, because from their perspective, those kinds of demands were ridiculous and they had never seen them before. After all, everyone else doing programming, art, and whatnot aren't part of a union and are just subject to whatever their workplaces throw at them and this includes the publishing economic system in games that SAG-AFTRA is trying to fight (and AFM was trying to fight before them). In their eyes, why should voice actors be any different? The exception would seem unfair from the outside, and of course there's the obvious factor that they would lose money.
Now, that is a bad thing, yes. More disciplines in game dev/production need to be unionized because working in the game industry is pretty awful, working condition and quality of life considered. But as it stands right now, there isn't enough there for SAG-AFTRA to succeed in a standard union negotiation type deal, so their approach is... well, inefficient.
That's because the AFM is run by someone who doesn't understand the game industry. Mr. Austin Wintory himself: