An RDBMS can handle considerably more tasks due to the power of the relational tools, and there’s a huge existing library of tools built around that model. That means that there are far more projects which can be implemented using only an RDBMS than only a KV store, and in the modern era both are easy cloud options with SSDs pushing caching past the level that most projects will ever need.
There's even databases that are focused on increasing the capabilities of rdbms.
Point is that sqlite could take people way further than they think. Postgres incredibly far. By the time you need a caching layer where key value stores shine best you have many options you can do and resources to do them.
