They do it because it benefits them too. OpenStreetMaps links with WD, GLAMs link with WD, journals/ORCIDs link with WD, all sorts of other data archives link with WD. Whoever is not linking with may see a crawler pass by to collect license-free facts.

Also, I just checked: WebMD is using a ton of embedded RDF on each page. They understand SEO well as you said :)