Because that business model is profitable, and entertainment is a business like any other. If you don't like it, you are free not to play it. Besides some potential for gambling addiction (about which state lotteries are a far more important target for your ire if that is your concern), mobile games produce no externalities whatsoever.
I continue to be completely mystified with the concept of outrage about video games. If you want to be outraged about corporate behavior, there are a multitude of examples of corporate malfeasance with actual negative externalities, such as the coal industry's contribution to climate change.