I would say you get into the habit of fitness for life, by finding the right sport or fitness activity for you, and that will be different for everyone.
Therefore, the approach taken by most schools, of offering 1/2 a term of several different activities (incl dance, gym, trampolining, etc as well as swimming, racquet sports, team sports and things like trampolining) gives children the opportunity to try all sorts of things and means they are more likely to find something they do enjoy.
Plenty of people don't enjoy maths, but they all still have to do it.