Comment by JanisErdmanis

It’s about coupling and being able to maintain that in the long term. A narrow focus helps to test each individual unit in isolation from each other. It is true that a database appears to be a single datastructure with hundreds of methods from the users perspective and that is fine, because someone else engineered and tested it for you. However if you were to look into how a database is implemented you would get to see the composition of data structures, like btrees that are tested in isolation.