I've seen this question a LOT. Why can't someone make an adult diaper that looks, feels, and most importantly works just as good as a baby diaper.
First, copyrights and patents. Baby diaper manufacturers want any advantage they can hold on to, and not let anyone else use their patented designs. Baby or adult.
Second, I came across this image. This is just how different our body shapes are from newborn to adult. So a diaper shaped for a baby could never "scale up" and work for an adult.
https://9gag.com/gag/ajXG1MG?utm_source