You've pinpointed what sent UML off the rails. CASE tools were going through one of their hype phases about the same time UML was getting attention. The CASE people latched on to UML as the notation that would enable automatic code generation and round-tripping, and that led to the contortions you mention.

We still have diagramming tools that create and work with the useful parts of UML in reasonable ways, we just needed to jettison the no-code weight that was dragging the notation into dead ends.