Side loading is a threat to Apples control over all iOS and similar AppStore devices. If I didn’t want to comply with the very far-reaching requirements for e g in-app stores, I could just ask my users to side load my app. If for example Audible did this, it’d be a real threat.

Chief among this is the requirement that all in-app purchases have a 30% tax by apple.

Which is why you can't buy books in the kindle ios app[0] among others.

[0]https://www.techspot.com/article/1597-how-to-buy-kindle-book...

Still waiting on regulators to slap Apple for that obvious anticompetitive behavior.