Strollers

When did parents decide that children can't walk? We went to the Zoo the other day and there were soooo many 3, 4, and even 5 year olds riding in strollers. It is really sad. Elle hasn't been in a stroller since she was about 15 months, because she wanted to walk, and wouldn't stay in it. Are kids just out of shape nowadays or what? Is this some new parental obligation I don't know about?