Scott Ross, founder and former CEO of Digital Domain, spoke for years about this. The issue is there are just a handful of film studios capable of keeping a large VFX facility busy, so they have a lot of leverage when negotiating contracts and setting prices.
Because that part of the film process can run offshore, VFX studios find themselves fighting for these contracts and/or accepting unadvantegeous terms. I remember him saying that film studios can strong arm a VFX facility to open in a place under heavy tax breaks, and they are expected to 'forward' the cost savings.