Why do so many programs use rational databases instead of loading the data during startup and keeping it in memory? Especially for smaller datasets I would think, that a database adds unnecessary complexity and overhead. Also, a lot of data can be saved using modern RAM and when using an in-memory approach, optimized data structures can be utilised to further improve the performance
RDBMS do not imply persisted data. There are plenty of databases which support or are designed explicitly to work as in-memory data stores. Perhaps the most popular one is SQLite, whose in-memory store is a popular choice.