A database, that should be highly available, that is on a node that's going down, might disrupt database clients and end-users.
Not to say that you can't host databases inside the cluster, you can place databases on more stable servers (tag them with "db", be careful when you make changes) and don't move workloads, or manage expectations with the database clients.
We did have redis in k8s, but the redis cluster our team needed was over 50 nodes and was experiencing all kinds of odd problems and our ops team insisted we move to vms. Performance was better. We didn't use disc, all memory. Wish I could recall the issues better.
so unless your devops/sysops/whateveryouwannacallitops has a lot of experience, it's just not worth it when things like RDS/ElastiCache exists.
i worked at a place that was 100% k8s, but all stateful things (redis, mysql, rabbitmq) were either hosted or in VMs.