If you still have not determined for yourself a powerful motivation that would become a pusher for sports, perhaps you just were not interested in what can happen to the body if you ignore its need for physical exertion.
First, without systematic sports, the level of self-discipline decreases, a person becomes a hostage to bad habits and unhealthy nutrition.
Deficiency of physical training, as a rule, leads to the disturbance of normal sleep. Unearned energy causes insomnia and does not allow to have a proper rest. Lack of exercise leads to oxygen starvation and rapid fatigue.
An indispensable indicator of adulthood is a slow metabolism. And in the absence of physical exertion, it arises in the body faster than with regular training. The lack of sports activities leads to the fact that the muscles gradually atrophy and increase the risk of injury. Addiction to depressive states occurs when there is no reason to be proud of your own victories. If you regularly observe the improvement of your own abilities in sports, then it certainly raises the mood.