How come the shape of glan of males across the world is identical whereas other features of the body varies to great extent?

It's not identical. They're all pretty similar, but there is a little bit of variation.

Lots of things are very similar in human anatomy; for instance pretty much all of your internal organs.

circumcised penis = mushroom head

uncircumcised penis = taper shaped

They are slightly different but you right, similar enough around the world -- size and coloring are the main differences, though some do have a curve or bend others don't. The penis has one prime function and that is to be used for vaginal penetration-- so it evolved a long time ago to its optimum shape. Plus many genetic differences elsewhere- body size, arm length, body type (mesomorph, ecto- and endomorph) and such things as facial structure, skin color ect are all due to genetic breeding--- people of the same culture (including skin tone, body characteristics) tend to group together and breed together. but one thing that is not observed for the most part during considering mating is penis shape or size- that tends to not be a factor in most cultures when considering who your mate is. Today's European and American culture may be a bit different since premarital sex is so prevalent-- and sure, some girls who "test drive" a guy may decide to shop elsewhere for a partner if they don't find his length or thickness to their satisfaction. 50 years ago, there were not nearly as many "test drives' and more girls married as virgins (as did guys) so you get what you get down there.

actually, there are some variations in the shape of a **** head. Big mushroom and dolphin are a couple of variations.

Rest of the male doesn't matter.

Obviously it has actually worked for the last 3.8 million years.