I personally don’t see the argument for categorising it as immoral on the basis that’s it’s not useful. The same could be said about plenty of other enjoyable things.
However exploitation is clearly immoral. That’s where I have issue with gambling. Gambling operators don’t get rich thanks to the average users but because addicts give them much more than they should. That’s clearly immoral be it from a casino, a gambling website, or micro transactions in mobile game. Every companies which profit from that should be held accountable including Apple and Google which are clearly complicit.