As of 2017, no single graph query language has been universally adopted in the same way as SQL was for relational databases, and there are a wide variety of systems, most often tightly tied to one product.
I don't know how popular Graph databases really are, but this might well be the reason why Graph DBs hasn't been introduced within the Laravel Core. If there is no single language to talk, I can imagine it's very hard to develop support for it.
The bottom line seems to be a lack of adoption. Maybe you can persuade Taylor to integrate a default Graph driver in Laravel 5.7. To improve adoption rates ;-)