Heroin addiction is an addiction problem too... but TV addiction, like heroin addiction, seems quite widespread. (Average screen use is now something like 8 hours per day, IIRC.)
There might also be externalities. Suppose you want to play outside, but all your friends are watching TV.
There also are knife accidents, car accidents, swimming pool related drownings, email addiction, work addiction. These exist, but what are the externalities in general?