One potential difference is that the input data required to render a single frame of a high end animated or VFX movie might be several hundred gigabytes (even terabytes for heavy water simulations or hair) - caches, textures, geometry, animation & simulation data, scene description. Often times a VFX centric cloud provider will have some robust system in place for uploading and caching out data across the many nodes that need it. (https://www.microsoft.com/en-us/avere)

And GPU rendering has been gaining momentum over the past few years, but the biggest bottleneck until recently was availabe VRAM. Big budget VFX scenes can often take 40-120 GB of memory to keep everything accessible during the raytrace process, and unless a renderer supports out-of-core memory access, then the speed up you may have gained from the GPU gets thrown out the window from swapping data

As a specific example, Disney released the data for rendering a single shot from Moana a couple of years ago. You can download it here: https://www.disneyanimation.com/data-sets/?drawer=/resources...

Uncompressed, it’s 93Gb of render data, plus 130Gb of animation data if you want to render the entire shot instead of a single frame.

From what I’ve seen elsewhere, that’s not unusual at all for a modern high end animated scene.

To re-enforce this, here is some discussion of average machine memory size at Disney and Weta two years ago:

https://twitter.com/yiningkarlli/status/1014418038567796738

Oh, and also, security. After the Sony hack several years ago, many film studios have severe restrictions on what they'll allow off-site. For upcoming unreleased movies, many studios are overly protective of their IP and want to mitigate the chance of a leak as much as possible. Often times complying with those restrictions and auditing the entire process is enough to make on-site rendering more attractive.