First, there is a ton of already built tooling for dealing with an RDBMS. This alone is why I think every project should default to using an RDBMS and only move if a good case can be made.
Second, like it or not, sql is the defacto language for querying the data. After 20+ years of writing software, I've used many languages, but sql (of course it has evolved) has been a constant across RDBMS. I've seen many service layers and front ends change over the years, all while sitting on the same RDBMS. Given the speed at which tech moves, it's pretty amazing when you think about it.
Finally, for things like graphs or json, it's often the case that I only need handle it in addition to my existing relational datastore. In some cases it might make sense to stand up an entire system for the use case (like a solr server for document searching), but many times I just need to store a bit of json or I have one or two graphs that need to be represented. In these cases, standing up an entire separate system adds unneeded complexity.