Occasionally, it’s not just the song that calls to our attentions, it’s the visuals in the sequence itself. That’s not to say that the song itself is an afterthought, but I theorize that once an episode of anime is over, we have a tendency to already be on our way out so to speak. It may explain why a lot of times the visuals of an ending sequence seem very haphazard and half-baked in comparison to the comparably more upbeat opening pieces of many shows which are priming you through both songs and visuals to internalize the mood and tone of what it is you’re about to watch. I will be the first to admit, that if the ending song doesn’t immediately grab my attention as an episode is finished, I’ll be less likely to pay attention to the visuals themselves.
But hey, sometimes the strength of the visuals themselves are enough to cause them to linger.