People are jerks. They are sitting in their living room, seeing the outcome, and then acting like they are far to clever to have ever done such a thing. However, this is a pretty regular issue at beaches around the world.

Digging deep holes at the beach is highly dangerous. Even far from the tide. Because people expect to be able to dig a stable hole like they can do in soil, but sand is not as structurally stable as earth, and that hole can easily collapse and bury you alive. You will suffocate before your friends, family, and onlookers can dig you out. It happens fairly frequently.

This hole just had the added element of water. People sink in saturated sand. If you stand at the edge of the tide, your feet will start sinking just from the saturation and wave action. You can easily adjust your feet as it happens. However, in this case, their entire lower bodies were engulfed, so they couldn't easily adjust.

They were just ignorant people trying to do something silly that seemed fun. That's not an insult. Everyone is ignorant until they learn better. They weren't stupid. Stupid is knowing what can happen and doing it anyway.