JSON is just JSON, any system that parses JSON can parse JSON, be that SQL Server, powershell or any other tool you might be using.
JSON has the fundamental problem of being hierarchical.
Consider the following
How do you write a query that shows which customers bought particular product groups? You have to navigate down the customer hierarchy and then back up the product group one. In the relational model no navigation is necessary. This is one of the reasons that relational DBMS swept aside the hierarchical approaches of the 60s and 70s.
There is no standard for what is relational meta data or how the data itself should be sent or for that matter any requirement that the data have relationships defined in a standard way.
There is no syntactic standard, that's true, though all you need is a standard interface that implements the relational operators (restriction, projection, union, intersect and minus), you are then free to send the data physically however you want.
A short while ago XML was all the trend, now it's JSON. How long will JSON last before someone comes up with something new? The relational model has existed for more than 50 years and shows absolutely no sign of going away.
Relational databases are great for storing large amounts of well structured data that you want to be able to efficiently slice up and query into small chunks.
It is perfectly possible for data to be represented relationally without it being physically stored. You can store the data physically however you want, the relational model is solely concerned about the logical level and has nothing to say about storage.