Because in practice it showed up for a period of time as a common thing in spam-senders. They were trying to maximize throughput and minimize software maintenance costs, so they leave out things that the spec says are optional. But that makes "a commonly-implemented optional thing was left out" into a stronger spam signal.

Is it still a strong spam signal? Hard to say. Sources disagree. But as with laws, heuristics, once added, are often sticky.