From the projects I've been involved in, the reason is simply that we don't want to have 2 persistent storage systems. There's a need for a fast cache system, and there's a need for a reliable – as in certainty above speed – database. The former is usually Redis, and the latter most often needs to be a full-blown SQL database to handle the required complexity of larger applications.

It's just easier to have one single source of truth. Please don't change Redis into a large SQL database. :)