> How does this happen?

Simple. Evolutionarily, the dumber you are, the higher the risks of dying, because you do stupid shit. Inversely, the safer the environment, the dumber people become, because not doing stupid shit stops being evolutionarily advantageous. Nature is pretty brutal so there is evolutionary pressure not to be dumb, but in modern civilization we have pretty much infinite resources dedicated to safety, which means there is a feedback loop "people are dumber -> more safety measures -> people are dumber -> more safety measures".

When you go to poor countries you'll realize just how much knowledge children posses on the topic on how not to die.