The OS already has an extremely fast, reliable, key-value db built in --- it's called the "file system".
How much data are we talking about here? Gigabytes or megabytes? In many cases, a 3rd party db is like using a sledgehammer to squash a gnat. People use it because --- well, other people do it and that is how it's supposed to work.
I have written medium scale multi-tenant web sites without a db anywhere in site. And I've had "professional" web developers who've seen the result tell me it's just not possible. When a sledgehammer is all you know, everything looks like a rock waiting to be smashed.