Wikidata seems to redefine common RDF properties such as:
What's the motivation behind this? Why not just use the RDF properties, making it more similar to other knowledge graphs and therefore easier to query?
Following up on Stanislav's and AKSW's comments, I did a bit of research.
As far as I understand it (improvements/corrections are welcome), Wikidata has its own data model (it is not natively represented as an RDF graph -- quick introduction here). For example, Wikidata statements are ranked, they can be linked to references, etc.
These features can be replicated in RDF (using, for example, RDF reification) but that can be somewhat verbose and/or complex. As a result, Wikidata defines its own properties and classes, while still linking them to the equivalent RDF constructs.
Worth reading: