Pixar has open sourced several projects at GitHub, under the Apache license. Most significant is probably their OpenSubdiv software:

"OpenSubdiv is a set of open source libraries that implement high performance subdivision surface (subdiv) evaluation on massively parallel CPU and GPU architectures. This code path is optimized for drawing deforming surfaces with static topology at interactive framerates.

OpenSubdiv is an API ready to be integrated into 3rd party digital content creation tools. It is not an application, nor a tool that can be used directly to create digital assets."

https://github.com/PixarAnimationStudios

Maybe it's due to Apple's (Steve) history with Pixar ?

It's more likely because of Pixar's longstanding practice of keeping its core technologies openly available (Blender is rife with technologies originally developed by Pixar gurus, not the least of which is RenderMan). Disney may be a huge company but Pixar is the only part of it that has ever won an Academy Awards (it has about ten as I remember) except for a single award for Beauty and the Beast. A Pixar guru also runs both Pixar and the entire Disney Animation Studio. Jobs never had a reputation of being generous with patents. It's also just a smart move in general, people are tired of application-restricted file formats in this field.

If they did that nobody would support it. That wouldn’t be very helpful.