Contrary to what a lot of people outside the business think, the VFX industry does not work like an assignment that can be given to an individual and delivered as a finished product which is integrated into the final release after passing testing.
A better analogy is when rough shots are entered at one end and go through a number of stages such as modeling, BgPrep, Dust Busting, Tracking, Animation, Lighting, FX, Compositing and so on. An average shot is worked on by 10-12 people, each of whom devotes up to four weeks or more, each on it. Additionally, there are people responsible for the production pipeline ticking, making sure that the IT systems, render farms and networks are working, etc. This doesn’t include the support functions like Accounting and HR. VFX is compute intensive.
A single VFX frame can take up to 12 hours or more to render depending on complexity. Most shots see at least 20 iterations. Building and running a render farm which can churn out this much compute power is not cheap. Moving on to storage, studios need to maintain backups of all this data, which definitely comes under the expensive areas. This gives a clear idea of the huge cost behind all the roses and sunshine you watch on your screens.