Why? It's the essence of "Simple Made Easy": you don't have other code to complect with. You have a smaller interface, focused on a singular goal. When a library has to work as a standalone project, it can't be accidentally entangled with other components of a larger project.

Smaller implementations are also easier to review against malware, because there are fewer places to hide. You don't have to guess how a component may interact with all the other parts of a large framework, because there aren't any.

There are also practical Rust-specific concerns. Fine-grained code reuse helps with compile times (a smaller component can be reused in more projects, and more crates increase build parallelism).

It makes testing easier. Rust doesn't have enough dynamic monkey-patching for mocking of objects, so testing of code buried deep in a monolith is tricky. Splitting code into small libraries surfaces interfaces that are easily testable in isolation.

It helps with semver. A semver-major upgrade of one large library that everyone uses requires everyone to upgrade the whole thing at the same time, which can stall like the Python 2-to-3 transition. Splitting a monolith into smaller components allows versioning them separately, so the stable parts stay stable, and the churning parts affect smaller subsets of users.