This is possible because the more they are closer to their gadget, the more they feel that urge to open the app and gamble. This is not the same thing as a gambler who gambles often offline, that alone will limit him automatically.
It's true but it can only work if the person gambling offline doesn't have a land-based casino or a betting shop near them and they need to travel from their place to somewhere far away to gamble which they might not always do because you wouldn't want to travel more than 20km just to gamble with $20 or something which a person wouldn't bother doing if they are gambling online since they can do it from the comfort of their home, sitting at the couch having a snack.
However, if the person has a casino or a betting shop within walking distance from where they live, it might not have that big of an effect on them and their gambling addiction because whenever they have some money, they can walk into the casino or the betting shop and gamble it away without any problems or a second thought.
I think the truth of the matter is that, gambling have been made really easy for everyone, whether you are gambling offline or online, it's pretty easy to gamble, and this is because, for those gambling offline, it's easy in the sense that, they don't have to travel far anymore as offline casinos have become a very popular and as well as a very lucrative business, we have a lot of them around us and not far away anymore, and even a gambler lives in a remote area where casinos dont find good for their business, there are actually betting agents usually stationed on those kind or type of places, so, one doesn't really have to travel too far to gamble anymore, just walk into the betting agents shop and state what you want, and that bet will be placed for you ASAP.
And for those who gamble online, it is the most easiest this days, and this is why I personally believe that online gamblers are the most addicted when it comes to gambling, quitting is always hard as long as the gambler have a mobile device on his hands with an internet connection, and where its even harder is that, most gamblers still do other meaning and productive things online, like making money through freelancing, trading and so on, so, it is not easy for a gambler to give up his or her device and stay off the internet all in the name of trying to break free from gambling addiction.