Google's knowledge graph is much, much better than any of the open data competition because they have done the work to make it consistent (not in the ACID sense but in the completeness sense)
For example, Wikidata appears good on the surface, but as soon as you try to build against it you find huge holes in the data.
As a more specific example, the most common example you will see on Wikidata is "list the cities with a female Mayor in order if population." Great, except it turns out that many (most?) cities aren't marked up with the attribute that makes them considered cities for the purpose of that query.
Knowledge APIs add typing to search. That's really important because it let's you disambiguate queries well (Apple computer vs Apple fruit) and behave more intelligently based on that type.
Things like the DDG API (mentioned in this thread) don't do that. DBpedia/Wikidata/Yago do it, but so inconsistently that the benefits are hard to make useful (as you are coding for the multiple ways types are handled).