It probably is because wage tiers tend be set across the industry rather than being negotiated between artist and studio. Voice actors, for example, are separated into 3 tiers with strict pay scale differentiation and no residuals, so only a very few of them have negotiating power. Most anime VAs make their real money by doing work for games or corporate videos, where these industry pay scales are not institutionalized.
It probably is because wage tiers tend be set across the industry rather than being negotiated between artist and studio. Voice actors, for example, are separated into 3 tiers with strict pay scale differentiation and no residuals, so only a very few of them have negotiating power. Most anime VAs make their real money by doing work for games or corporate videos, where these industry pay scales are not institutionalized.