Probably because there is a nugget of quality and a whole lot of nuggets of attention attraction built into the game. Check out Vampire Survivors sometime, it's free on Android and it doesn't have ads (unless you go out of your way to click the button that says "view an ad"). And it was developed by someone who had previously worked at a casino.
It's the same reason somebody relentlessly checks Twitter, it triggers the same dopamine receptors.