Because *contrary to popular male belief* women do not select a man by the size of his penis alone.

A micropenis is well able to impregnate a woman.

But even more: evolution isn’t that precise. Genetics are convoluted and complicated. As a made up example imagine gene A makes you immune to malaria, gene B makes your eyesight fantastic, but having A and B together makes you have no penis at all. Those genes would still be somewhat competetive, because people who only have 1 of them have an advantage and pass the genes on, and women can savely carry AB without issues.

I.E. there is no single-purpose-micropenis-gene.

Because sperm can move on their own, and so penis size has no bearing on evolutionary advantage. As long as you get it in there, the little swimmers can handle the rest.

Well, they would have disappeared if they weren’t functional. But after quite a while, things like this don’t have a fixed amount of time to happen.

Basically, if your friend down there works, there’s no need for humanity to change that. You are still capable to breed.

Obviously, if girls chose who to have babies with based on the weenie size, there could be a problem. But that’s not the case.

Having a bigger penis doesn’t make you more *fit* in evolutionary terms, contrary to popular belief.

The size of your penis does not matter.

Also, men don’t walk around naked, and selected based by this quality. There are other qualities that women choose, since they can’t see their genitals.

, traits that are unfavorable and hinder reproduction tend to be phased out over time

However, the existence of micro-penises can be attributed to a combination of genetic variations and other factors that don’t necessarily hinder reproduction.

This is because a micropenis is not a simple genetic trait like blue eyes or a big nose.

Instead a micropenis will often occur because of a failure of the gonads to produce enough sex hormone for the penis to fully develop, and that in turn would be due to a problem with the pituitary gland or the hypothalamus.

So it isn’t like people with a micropenis are breeding true, instead it is simply that some people have genetics which make them somewhat more likely to have pituitary or hypothalamus problems that result in a micropenis. Those problems are very rare to occur, less than 1% of the population, and so they aren’t impactful enough to be completely eliminated through natural selection. Someone could have genetics that make them more likely to have a micropenis and yet it is overwhelmingly likely they still have normal genitalia despite that. All the equipment would work fine and until relatively recently everyone would be completely unaware there was increased risk!

A lot of things in biology are like this. For example some people have genetics which make them significantly more likely to have heart attacks than the population at large. Yet that trait isn’t completely removed from the population via natural selection because while they have significantly higher risk the actual prevalence of the issue is still very low. Even if they have three times the chance of the average person to have a heart attack, three times a slim chance is still very unlikely.

Sexual selection means that women will tend to choose men with larger penises. Women also select for other traits, as other commenters have mentioned. One point I’m not seeing mentioned is that historically speaking… sex is not always a woman’s choice.

Micropenis are usually caused by testosterone deficiencies, which can be caused by a variety of different conditions. Many of those conditions are genetic that just come from random mutations with no cause.

If based on natural selection, the other Redditors have answered it.

However if you are asking from a survival POV, my rationale is we (humans) tinker too much.

We have medications and procedures that allows us to prolong/sustain life, promote passing on of genetic diseases and defects.

It’s a slippery slope to start going down so will leave it at that.

I am however GLAD for the tinkering as it has prolong my life but flip side is my kids have a 50/50 chance of having it which will not be known until exposure happens.