Watching sports is a form of entertainment. You could say most forms of entertainment are a waste of time because the person does not accomplish anything. Personally I started exercising while I watch TV so watching TV is not a waste of time. I accomplish something. When I play games I also like to accomplish something. That is why I like ones the exercise my mind or body.
In my opinion just sitting around and watching something for a long time is a big waste of time. However you could accomplish something while watching sports. You could be inspired to play sports or exercise. A few people have gotten rich from watching sports. For example a person watched a boxing match, wrote a boxing movie, sold it, starred in it, and became a rich actor. It is also not a waste of time if you are watching kids playing sports.
Most of the adults I know that watch sports on TV don't play sports but they spend a lot of time sitting around doing next to nothing.