The reason we have tailbones is that they have zero potential effect on our health. Thus, there is no reason why individuals that expressed the mutation of having larger or smaller tailbone would pass on that mutation at a higher rate than individuals that had typical tailbone. We do not have arbitrary, gratuitous features like foreskins. The penis has the form that it does for a reason. If no foreskin would have been better, then the genes that caused foreskin would not have dominated the gene pool and vice verse. This is much like how species of fish that live in caves do not have eyes — although fish generally have eyes, eyes would be a liability (potential source of poorer health) for fish that live in absolute zero light conditions, thus fish that were mutation-ally born without eyes had a health advantage that, no matter how tiny of one it was, the mere fact that it was any advantage at all, by the exact nature of evolution, meant that that gene was fated to dominate the gene pool.
The reason men have nipples is plain and simply that they are of the same species as women, women need to have nipples, and the two sexes are differentiated not by fundamental major physical differences like the existence of nipples, but rather only in the form taken on by these features - by effects that occur as the result of hormones that orient the fundamental physical features in different directions. Breasts are the same between men and women except that female hormones have caused them to develop the capability of lactating (which men can do too if given female hormones).
Anyway, the origin of circumcision is religion and the traditions and cultures so derived/passed on, not anything else, so that’s the explanation for why it is done at all. Though there are apparently various theories on who started it first and why, the theories are primarily closely related to religion. For example, as sacrificial act or act of submission to deity. Of course, now that we know more and more about all the negative effects of the act, circumcision is rapidly becoming less and less common.