Many people have consumed alcohol for decades without problem, too... Can you simply ask an alcoholic to stop? Some people are genetically predisposed to addiction. These vulnerabilities interact with environmental factors (e.g. adverse childhood experiences) to increase the risk of developing an addiction in a given individual. This probably includes behavioral addictions, such as sex addiction.
The word "addiction" is pretty clear to everyone, but because scientists/doctors like to argue about words and classification, it is also somewhat controversial. For instance, the DSM-V speaks of "substance use disorder" or "alcohol use disorder", not "addiction".
When it comes to sex addiction, while it is not officially recognized in the DSM-V there is abundant scientific research, from studies of clinical populations to brain scans, on this topic. For instance, the International Classification of Diseases (from the WHO) has an entry for "Compulsive sexual behavior disorder". Here is the definition (read carefully):
What has happened in recent years is that the internet, smartphones, social media and various apps have boosted, by their very nature, the addictive potential of many behaviours. That's why we end up with so many people glued to their phones. There are now clinically recognized addictions regarding social media , online gambling and online shopping, to name a few. This has, obviously, wide-ranging impacts on society and human relations, which I have discussed elsewhere.