In my ears, knowledge graph sounds a bit grandiloquent. I do not have a definition, but I know that when talking about knowledge as it is embodied in people, it's quite a subtle thing, hard to formalize and to be honest, something relatively rare.

Why can we just call these things fact databases?

Add. Knowledge evokes a lot of other associations as well, for example that what we are able to know changes over time. That a time has a certain underlying grid, into which certain factual stories appear and later disappear.