As embryos, males and females have similar tissues and body parts. The embryo follows a 'female template'. Jehovah God designed it so that when humans form in the uterus, the sex doesn't become determined until later on in our formation. Because nipples are there before this process begins the nipples stay.
"Oh Jehovah...I shall laud you because in a fear-inspiring way I am wonderfully made." (Ps. 139:14)
For more, see
Why Do Men Have Nipples?
The Human Embryo's Sex Differentiation: Why Do Men Have Nipples?
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