why do girls have zip in their paint?

in all "normal" human beings, the hip area is wider than the waist (where the jeans sit), in order to wear them, the opening should be wide enough so that the person can slide it up his/her hip and pull it up to the waist. if there is no zip, the waist opening has to be "atleast" as wide as the hip, since if it is less than that, your jeans wont move beyond your thighs. if it is as wide as your hips, without a zipper, then it would slipe up your hips, but would not stay on your waist, it would slide down. so the solution is to have a zipper or an elastic. when we wear it, the open zip or stretched elastic helps it to cross the hips, and when it reaches the waist, the closed zipper and unstretched elastic helps it to stay there! so to pee or not to pee is not the issue! if you want your pants to stay in place we need a zip/elastic!

No reason as there is no reason that why do men have zip in their pants. Is there any???

why do you think it should not be there.

I don't do painting. What do you mean by zip in paint? zziping 2 pices of paint? do you mean oil paint?

For the same reasons that men have zippers in their pants. Women need to remove clothing and pull pants down to pee, just as men need to unzip to do the same... We just have to pull them down further! Zippers are also a fashion thing. Some women's slacks and pants have zippers, some have buttons, and others have elastic waistbands.