It's really not!
There's a reason why Tinkerpop is what most graph databases standardize on, and why things like Neo4J, DGraph, Caley, TitanDB etc (ie, all the graph DBs which people use when they want to build something and not "do the semantic web") don't use SPARQL.
Tinkerpop is a Java API, not an independent query language, and RDF data (while it is a way of modeling a graph) is not the model of most graph databases (it's a lower-level model than most graph databases expose, and is about as far from them as it is from the table model of SQL databases.)
An optimal Java API for graph databases with a more typical model is not an optimal query language for RDF data, for pretty much the same reason SQL isn't.
Now, what you describe is probably a sign may that RDF isn't the right exposed data model for many use cases (EAV-style representations are often used for deep internals, but there's probably a reason that outside of RDF most systems which use the model internally expose something more similar to the conventional relational or graph model to application developers), not that SPARQL isn't the right language for querying RDF data.
Gremlin is very widely supported across graph databases.
what you describe is probably a sign may that RDF isn't the right exposed data model for many use cases
Well. that's exactly what my claim is, so that's good!
not that SPARQL isn't the right language for querying RDF data.
Have you ever tried one of the alternatives? Try GraphQL on DGraph (Or Gizmo/Gremlin on Cayley) against a Freebase or DBPedia import. That's exactly the equivalent of SPARQL against RDF, and it's so much better.