Historically the overhead of SQL put a low ceiling on the potential performance of simple queries. Which is exactly why KV databases appeared in the first place, because a scalar value get in most RDBMS systems barely hit 1000 results per second, and that is being optimistic. That overhead was never optimized away because the classic use of the RDBMS was query conversations where each interaction was very expensive (e.g. large, complex queries), so optimizing it wasn't important.
But, thanks to competition, most database systems have gotten fantastically more performant. Even using the TDS interface, I can get close to Redis performance with SQL Server now, while it was beaten by one if not two orders of magnitude a few versions ago. And in this case MySQL added the ability to use a much simpler, lighter memcached API in from of MySQL, for obvious reason -- the underlying database system is fast enough that it is of value.
I should add that almost everyone who has commented thus far completely misunderstands what this is taking about. It is the Memcached API atop MySQL. Meaning you can talk to your database server through that API from any client programmed to talk to a memcached server. And that same data that you access via memcached (reading or writing) can be accessed in your normal SQL. Which is pretty cool.