As someone who has played a decent of competitive games in the past, it is less about them being extremely serious, and more about how much people cheating ruins the fun for everyone else.
For example, a soccer Sunday league might not be that serious, but you still wouldn't want someone frequently running on the pitch with a golf club and whacking the ball into your goal with no consequences.
For example, a soccer Sunday league might not be that serious, but you still wouldn't want someone frequently running on the pitch with a golf club and whacking the ball into your goal with no consequences.