According to the Visual Effects Society 43 of the top 50 movies of all time are effects driven. Visual effects artists are the goose that laid the golden egg for studios.
Studios love this group. Why? Because visual effects artists are incredibly creative individuals with few demands after rates have been paid. Compare this with actors. Their contracts require ongoing financial responsibility. Their contracts are bullet-proof and have stood the test of time. Just try paying a Hollywood actor late! Their contracts include waivers, restrictions, riders, fees, insurance coverage, limitations, meal period violations, rest periods, penalties, holidays, percentages of sales, commissions, salary, bonuses, agency fees, pensions, and rebroadcast fees.
A lone visual effects professional has very little bargaining power. The lack of structure of a master agreement allows studios to pocket money that would otherwise have been negotiated in the form of benefits and residuals paid to the artist. While other groups require an ongoing fiduciary responsibility visual effects workers have given a pass to the studios.