The idea that filesystems are not just a flavor of database management systems was always a mistake. Maybe with micro-kernels we'll finally fix this.
1. Distributed filesystems do often use databases for metadata (FoundationDB for 3FS being a recent example)
2. Using a B+ tree for metadata is not much different from having a sorted index
3. Filesystems are a common enough usecase that skipping the abstraction complexity to co-optimize the stack is warranted