Pressure equalizes as they descend slowly. It's different from descending 4000m inside an armoured can and have it all released upon you in a split second.

Humans are less compressible than an empty steel tank (being mostly water) and when they dive they don't teleport from 0 to 10 meters in an instant.

Well, sea level is at 1 atm already, so an increase to 2 atm isn't immediately catastrophic. However, if you took that tank filled entirely with sea level air and shoved it down 30 feet under the water the same thing would happen. It instantly crumples because its hollow. We're full of water with some air in a couple places. We'd crumple too if we were just a layer of skin over an air sac. But as it stands, the human body can withstand that pressure alright