Why were we not designed to grow a tooth when we lose one? Why can some animals regenerate body parts but humans cannot? If we are the pinnacle creatures of "design," the designer didn't think ahead much.

To a degree they are very clever products of evolution. Most male fish, amphibians and reptiles have internal gonads, but because sperm are more easily produced and more easily stored at low temperatures and because mammals, being warm blooded, have generally higher body temperatures for longer periods of time. It was needful for nature to find a way to keep the gonads cooler, so they evolved to be on the outside, and with a very large blood supply so that they could also be raised or lowered to the optimum temperature if needed, hence the large blood vessels, especially on the outside. ( How birds manage I do not know. )

But putting them outside of course exposed them to danger, so they were given extra strong and numerous pain reception, and males were given strong instincts to protect them. Since any damage to them, could harm a males role in evolution greatly, and was therefore not favoured by natural selection.
Moreover, in most male mammals they are usually equal in size and placed side by side, but in humans it is quite normal for one to hang lower or be smaller than the other, which could well be, many think, to lessen the chances of them being trapped when walking upright, because to do that well requires the legs to be much closer together, than those of quadrapeds whose legs are often wide apart, which makes them more likely to be trapped.

So it is possibly, all very clever, but also mainly makes sense from an evolutionary point of view.