First, I'd say the key value store is actually older than the relational database, by at least a couple of decades. Of course, not the current ones, but we're also not using the relational database from the 80s so ...

Now to try and answer your question, I think a relational database is a lot more complex than a simple key value store, but if it's a good implementation, it will hide this complexity from the user, and using the database will be rather simple. It can be as simple as a key value store since you can just use a relational database like one. Note that this hiding of complexity is by design, it is one of the main goals of the relational model, it's not accidental.