Because Facebook announced publicly that Apple's privacy policy changes would cost them $10 billion[0], putting them on the record as having strong motive to avoid the App Store.
0. https://www.cnbc.com/2022/02/02/facebook-says-apple-ios-priv...
0. https://www.cnbc.com/2022/02/02/facebook-says-apple-ios-priv...
Additionally, iOS users' attention has traditionally been worth more to advertisers than that of their Android using counterparts because they buy so much more.