What makes this possible is Cargo's "workspace" feature, which is relatively new enough that it's not hard to imagine that some older projects may simply predate it.
Would something like git subtree help here? Allow people to continue working with their individual repos but present a unified view for outsiders to follow along? We could automate this, right?
Cargo workspaces are more of an optimization in that they let all the crates in your project share the same output directory[0], so built dependencies can be shared among every crate, as opposed to each crate needing to (re)build all of its dependencies independently.
This helps to reduce compile times when working on projects where you frequently switch between crates.