Online gambling just made gambling easier and more accessible than visiting physical shops, I don't know of oys right to say that it leads to gambling addiction because more people having access to gambling in an easier and flexible fashion does not not amount to gambling addiction. Although it has the tendency of making people gamble beyond limits since the deposit method is easy and smooth but I don't believe it leads to gambling addiction.
It is just statistics i think. The same percent in online and offline casino, but online gambling more accessible and more people are in it. As the result we more often hear stories about gambling addicts. If we calculate gambling addicts one by one - in online casino we will count more addicts. But if we compare percent of gambling addicts in online and offline casinos - i think it would would be the same.
Online gamblers are always prone to addiction because they gamble alone. In a typical offline casino, if you spend too much time there, other gamblers will begin to mock you, and such gambler will begin to limit how frequently he goes to the casino in order to avoid those conversations. However, in an online casino, no one will come to save you until you show signs of addiction, at which point a lot of harm has already been done.