I believe it's heart/stress related issues as well...
Out of Air is relatively low on the list of COD for dive accidents. The vast majority of accidents are caused by human error though, frequently coupled with panic. Do you think that a beeping computer would be comforting, or a stressor to add to a panic situation?
Do you honestly think that this idea of having annoying alarms is new? If it was a good idea, don't you think think that everyone would embrace it, and that manufacturers would place annoying alarms that go off all the time, or would the migrate away from that, as over the many years, people find that alarms aren't a preventative measure.
Sound underwater travels faster than on land, and directional sound is very difficult to discern. If everyone had alarms on their computers, everyone would stop and get all flustered when someone's alarm 20' away went off, and they think it's their own problem.
The simple solution, is to be an attentive diver. Check your gauges regularly, and not run out of air.
Divers need to take responsibility for their own life, and stop looking for someone else to do it for them. Alarms with sounds are crutches and an excuse to not pay attention to yourself on your dive... They are not aids.