Right now the term platform engineering seems to be a reflection of a collective intuition that software engineering teams need to have common patterns to simplify operating software.
I think it stems in large part from the complexity associated with running software in a deployed environment. Often, every system, from development to deployment is mired in process.
Tools and concepts have emerged to deal with _pieces_ of the complexity, but it seems demand is growing for a holistic approach.
Full disclosure I work for a company [1] solving this very problem. We are currently putting together a website that outlines our observations and conclusions about what a developer platform/platform engineering is.
1. https://noop.dev