Because a key-value store is a foundamentally simpler data structure (it's an hash) than a relational database, which tracks the relations between different data types. If you make an advanced use of the key-value store, you have a lot of logic in the application (for example to key management, cascade operations between related data...) which a relational database should do for you. It's not fair because there is a development cost in using the key-value you are ignoring.