Most large monorepos simply are not on git. Google has Piper, Yandex has arc, Facebook has eden (which is actually semi-open-source, btw!), some companies use Perforce and so on.
I'm very curious how it performs compared to EdenFS: https://github.com/facebook/sapling/blob/main/eden/fs/docs/O...
Dev Drive seems to be a special type of disk volume with higher reliability or something for dev-related workloads.
EdenFS is "Facebook's CITC", i.e. a virtual filesystem view into a remote version-control system.
"Monorepo" is a culture around having a single branch with a single lineage, and not developing anything in any isolation greater than a single developer's workstation.
I agree that Git is not very adequate for large monorepos, but I'd say that most open source projects are on Git, and most of them are trivial monorepos.
Microsoft's blog posts have indicated a move to use something as close to off-the-shelf git as possible, though. They say they've stopped using VFS much and are instead more often relying on sparse checkouts. They've upstreamed a lot of patches into git itself, and maintain their own git fork but the fork distance is generally shrinking as those patches upstream.