Why can a Cargo package only have one library target?

Cargo is primarily a package manager. Thus, the primary role of a package is to define a library.

When we use a crate as a dependency, we only specify the package name in our Cargo.toml. Since there can be at most one library, Cargo doesn't need you to specify which one to use. If it were allowed to define multiple libraries in the same package, then we'd need to specify a way to define dependencies between them, so you'd have two ways to declare dependencies (external packages vs. internal crates), making the system more complex.

On the other hand, adding a dependency that doesn't provide a library doesn't make sense, at least not with Cargo, since Cargo only cares about the library target in that context. Thus, there is no reason to limit the other types of targets (binaries, examples, tests, etc.) to one each.

I would expect that a cargo package can only have one library target because a library crate is by definition a collection of items (functions, types, traits, macros, values, etc.) while a binary crate has only one externally visible thing, a main entry point. Consequently, while the library crate's name is merely the root module within an hierarchy, the binary crate's name is the only thing.