People with money overwhelmingly buy iPhones. $25 is a pittance for anyone who can afford an iPhone. Anybody who uses Anki seriously gets far, far more value than $25 out of it. I used it for well over 200 hours before I stopped and I know I’ll go back to it again.
The Android version is not maintained by the developer of Anki. The iPhone app is. He chooses to charge for the iPhone app, which enables him to make a living making tens of thousands of people’s lives better.
If you don’t want to spend the price of two pizzas on an app that the modal user will use for over a hundred hours don’t.