The most common high-level use of a DBMS is the pair "given this key, get me the record that is its value" and "store this record under a new key". It's not exactly equal to the usual k:v usage (because it uses meaningless keys), but it's very similar.

That said, I don't think this is the usage scenario most DBMS are optimized for, because despite being common, it's a very fast case, so there is relatively little to gain. At the same time, this is the only usage of k:v storage, so you bet it's optimized for that.