This is what people don't understand about these micro transaction games, they aren't like Spotify, Twitter, or Amazon where you can subscribe to the service or pay for a video in the browser outside of the App Store.
They want the convenience of quick in app purchases because that's how these companies make money. You want this cool skin, extra pack, or special deal...buy now...like a casino with flashing lights. Gacha games need that in app purchase ability because it strips away the thinking/reasoning element much like casinos do.
In app purchases make them lots of money, they just don't want a portion of that money going to Apple.