A normally conscientious mother at a pool party thinks “someone else is watching”, no one sees or hears her child fall into the pool. A normally attentive father switches his attention to a flare-up on the BBQ grill his while his child sinks without a sound to the bottom of the swimming pool. A loving grandparent is attending the needs of one of her grandchildren, the other wanders off to an unsecured hot tub.
Supervision is vital, but it's near impossible to supervise young children every minute the day. That is why parents and caregivers should provide multiple layers of protection - pool fencing, self-closing self-locking gates, pool alarms, safety covers, in addition to constant supervision.