What do you mean other body parts are roughly the same?
Bones of every single person are of different lengths, thickness and density. Body lipid composition is different in every person, the colour of skin and hair is different in every person, hair texture is different in different people.
Not even identical twins, who share 100% of their DNA, are always the same physically.
Evolution revolves around survival and mating. Humans don’t typically mate based on genital sizes, they mate on muscles/fitness, facial composition, and perhaps more uniquely, personality.
In other words, there is no need for evolution to filter out smaller or larger penises or to have one size that’s best because people will factor many other things when deciding if they want to have sex. If, over the course of human history, women only had sex with men whose penises were say 5”, then you’d probably find more penises would be close to that. The standard deviation for penis sizes across men is more than enough to inseminate and thus never needed to be changed really
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