You know, for the kids.
Things like weight, encumbrance, hunger, thirst, fatigue, the usually limited availability of resources in any particular region of any particular world (such as iron or wood) to create different kinds of equipment or useful items for everyday life, etc., etc. Those kinds of things are too difficult for your average thirteen year-old to grasp, so we did away with that stuff years ago.
Even though according certain Terms and Services, kids under 18 shouldn't be playing MMORPGs without the consent, approval, and (hopefully) supervision of their parents. We also prefer that kids don't play during the school year so that they might find to do things like homework and studying. It might be better for kids to do things like read or play outside during their vacations, but we can't really prevent kids from making bad decisions that might have a drastic impact on their futures, or at least negatively effect the quality of their entire lives.