Google Research already maintains the Bullet Engine as well (although it mostly seems to be Erwin Coumans maintaing it)

Each have their own strengths, there is no "definitive" physics engine. Bullet is probably the most generalist engine, but it suffers a bit because of this approach (it's split between the old 2.0 C++ API vs. the new PyBullet API with vastly different functionalities)

One strong advantage of MuJoCo is its plain vanilla C bindings. Hardwiring an API like this to C++ or Python makes life much harder than necessary for users of other languages. Conversely, it's very easy to wrap a well-thought-out C API in your language of choice.

Super impressed with the data structures (and documentation) they've already released.

MuJoCo going open source is good for the open source community. PyBullet is build on top of a plain C API (wrapping C++ Bullet API). Note that there is also Brax and Tiny Differentiable Simulator, developed within Google.