Ah nostalgia... I recall wondering about this myself back in the early days of my career. However, that was before the current wave of document-based DBs (Redi

Ah nostalgia... I recall wondering about this myself back in the early days of my career. However, that was before the current wave of document-based DBs (Redis, Couch, Mongo etc.) so the meaning of a "database" was more targeted at RDBMSs. These days a lot of the reasons listed in the answers are equally applicable to, say, JSON files stored in Redis.