Disclaimer - I am not a student of psychology.
I'm currently researching Internet Addiction Disorder and online games for a paper I'm writing. Is there an actual reason why it's not included? Is it because it's such a new area of study?
There's debate over whether Internet addiction is it's own disorder or a symptom of an underlying psychosis. See this essay by the first man to publish the term: https://theconversation.com/is-internet-addiction-a-misnomer-55727