Debian is a large, complex operating system, and a huge open source project. It’s thirty years old now. To many people, some of its aspects are weird. Most such things have a good reason, but it can be hard to find out what it is. This is an attempt to answer some such questions, without being a detailed history of the project.
It’s a really good question which seems to have a complicated answer. This page here led me to this here (among other documents).
The short of it seems to be have that if you think of Rust in terms of “crates” instead of “libraries”, then it’s still possible to package in a way that conforms to Debian’s self-contained avoid-redundancy style, though the details of it seem a bit tricky.