I suspect that a large part of the problem is a combination of hearing loss and a lack of subconscious awareness of their phone. In both cases (and especially combined), they would likely not even hear the notification if it was at a volume you or I would consider reasonable. So, from their perspective, it is quiet. Any quieter and it would be too quiet to notice.

Of course, anyone capable of basic empathy would also realise that such a perspective is their subjective experience of the world and simply turn the volume off/down until later to avoid annoying other people. Sadly, many of their ilk seem to have forgotten the lessons they taught us as kids.