Interests, hobbies and other extra curricular activities are generally not included because they are usually not valid or reliable indicators of your ability to do the job. Some people believe that if a person is involved in certain types of activities outside work it indicates something about their personality or their leadership skills or some other traits. Most Organisational Psychologists will say that this is nonsense (but will use psychobabble in saying so) and can encourage type-casting and lead to erroneous assumptions. There is absolutely no basis for making a valid connection between what a person can do on the job and what they do outside work.
In any event, you usually have no way of knowing whether the people reading your resume will view your interests and hobbies as positive or negative. We therefore don't want to give them the opportunity to form a negative view at this early stage of the process.
This does not apply to graduate positions, where employers often want to know a person's interests and hobbies because there is usually little work experience from which to draw conclusions.
Employers may assume that people who have been leaders at school or university are somehow equipped to handle leadership roles at work. This is not necessarily true because many people become leaders at school or university because they have a personality and style that makes them charismatic and attractive. Whether they are effective in the leadership role is an altogether different question.