Independence is a skill that has to be taught.
Almost nobody is born with the innate knowledge of how to manage living on their own and attending to all of their basic needs. Once upon a time, every single one of us was pretty clueless on the matter. Some people also have the misfortune of never learning how to succeed at independent living, or it takes them a long time to grasp it.
A key part of parenting a teenager is giving them the tools that they will need to survive adulthood, and it could be argued that it is just as difficult as helping them hit their milestones when they’re young. Without this, they may struggle in the simplest of situations, especially if they are unexpected. It does not bode well for either parents or kids when they are operating on a level that is not going to serve them well in adult life.