Using traditional RAID (or moderate improvements on it, such as provided by btrfs and zfs) to provide drive-level redundancy seems like a waste of time when you also have to worry about redundancy between servers, racks, datacenters and regions.
All of the arguments for why it's better to have something like zfs handle RAID rather than layering something on top of a traditional hardware RAID controller also work for explaining why you should prefer managing storage more globally rather than layering on top of something like zfs—if you have the resources to develop and maintain a true "full stack" storage solution. Which Facebook/Meta obviously does, when they can do things like publish their own spec documents that SSD vendors design around.
All of the arguments for why it's better to have something like zfs handle RAID rather than layering something on top of a traditional hardware RAID controller also work for explaining why you should prefer managing storage more globally rather than layering on top of something like zfs—if you have the resources to develop and maintain a true "full stack" storage solution. Which Facebook/Meta obviously does, when they can do things like publish their own spec documents that SSD vendors design around.