Academics base their careers around citation numbers. You need publications and a high H-index to make it anywhere. Self-plagiarism reduces the effectiveness of that metric, which makes it harder to evaluate the actual impact of a researcher.

It should be no surprise that republishing in multiple journals was accepted in the pre-computer era, where citations were inherently harder to track (and thus less valuable as a metric).

Quoting Upton Sinclair, "It is difficult to get a man to understand something, when his salary depends on his not understanding it."