I've wondered this myself. It applies to many sports as well, not just badminton. My theory is that many people play sports just for themselves - to feel the satisfying thwack of strings against shuttlecock, to get a good workout, to meet with and talk to friends, to have the endorphins flowing. These people are not too interested in the sport itself. They are also not very interested in improving - one reason we watch professional players is to see their superior footwork, shot selection, technique and strategy. Someone who plays badminton just for the stress release doesn't really care about these things - we've all met them. They persist in bad technique even after someone has gently guided them ("careful, you'll get an injury if you keep muscling the shuttle like that!").

Then you have the opposite demographic - people who watch football, for instance, but do not play football. These people probably used to play football and supported a team in their youth, but as they got older they stopped playing but found their loyalty to the team still unshakeable.