Also because the install base for mobile devices is just about everyone everywhere, and yes, a fair amount of people, particularly young people, would much rather play something on their phone than a PC or even console.
The difference in potential customer base is orders of magnitude larger.
It’s the difference between a theme park and a casino. Both are legitimate forms of entertainment for many people, and both do need income to maintain their operations.
One of them charges for entry and then you enjoy the park, only paying for additional but ultimately optional things like merchandise or food. The fun ends when you decide to leave or the park closes.
The other is designed so you have to spend small amounts consistently, and it is designed in incredibly manipulative fashion, literally employing tactics that trigger addictive responses. The fun ends when you run out of money to spend, therefore compelling you to keep spending.
The people designing the theme park are designing something entertaining, the people designing the casino are perfecting a skinner box.
One is more deserving of income than the other.