Functioning adults struggle with Conservation

Conservation according to Wikipedia:

Conservation refers to a logical thinking ability which, according to the psychologist Jean Piaget, is not present in children during the preoperational stage of their development at ages 2–7, but develops in the concrete operational stage at ages 7–11.[1][2] Conservation refers to the ability to determine that a certain quantity will remain the same despite adjustment of the container, shape, or apparent size.

 So why do "normal," employed adults struggle with Conservation? I refer you to this Reddit thread: You can fit 60 grams of sugar into a 50 gram gummy bear.