IMO, it's because writers and actors form a seriously minor part of the labor needed to make a film or TV episode. From VFX to catering, there's a lot of labor

IMO, it's because writers and actors form a seriously minor part of the labor needed to make a film or TV episode. From VFX to catering, there's a lot of labor needed - much of which is highly specialized - that would cost too much to contract or develop independently.