One of the main differences I've heard referenced is that acting and being a movie star means that the work is fundamentally differentiable via the end-product, where producing software is meant to have the same outcome no matter who creates it.

That is just not the case with acting, where the end product being differentiable is part of the inherent value of the product.

Also, it's probably true that SAG's loss of industry power has very much to do with the loss of the power of movie stars in general.