CMMI stands for "Capability Maturity Model Integration". It's the integration of several other CMMs (Capability Maturity Models). By integrating these other CMMs, it also becomes an integration of the processes and practices within the model in ways that previous incarnations of the model(s) didn't achieve. The CMMI is a framework for business process improvement.
It is NOT an engineering development standard or a development life cycle. Please take a moment to re-read and reflect on that before continuing.
The business processes of developing engineered solutions are the focus of CMMI. This helps understand why it is most widely applied in software and systems engineering organizations.
But, where it is applied is a significantly distinct matter from being anything even remotely akin to a standard or certification mechanism for the engineering or technology required to develop products. If an organization chose to do so, CMMI could be applied in the construction or even media production industries.