The notation “org.jenkins-ci.plugins.workflow:workflow-support:1.13@jar” is known as an artifact-only notation.
From the documentation:
An artifact only notation creates a module dependency which downloads only the artifact file with the specified extension. Existing module descriptors are ignored.
Since you’re only downloading the jar file and not its associated POM file, you don’t inherit the transitive dependencies hence the need to manually define them in the Gradle script