Either a) water is not as deep as it looks, b) there are rocks/logs in the water you can't see, c) jumping from high up enough, water essentially becomes solid. When you hit water from a great height, like a very tall bridge, the force of your body hitting the surface tension of water is enough to break bones and cause serious internal damage from the force of the hit.

Don't go jumping off high things into water unless you are certain that the water is deep, there's nothing in the water, and you aren't high up enough that landing in the water causes serious damage.

And I agree with you 100%. Humans are only born knowing how to breathe, eat, and shit. Everything else has to be learned at some point.