Earlier quoted context omitted.
We are using text files. How many programs in a typical Unix installation need to communicate to a database? Next to none. How many are communicating via text files? Almost all of them.
How many programs in a typical Unix installation are complex? Not many. Most of designed around the concept of pipes and filters. They don't need a database; they are transformative maps. Complex systems often require complex data storage. If you have more than one process storing information into a file or set of files, you have to control that type of information. When faced with the option of writing complex, ACID…
I very rarely met things that really needed a database. And in most such cases it was not really a RDBMS. For example, a CAD needs a hierarchical database, relational does not fit there at all.