They are not allowing other marketplaces, or creators themselves, to run apps on Apple devices directly.

Why should they have to allow third parties to run apps on their platform? The fact that it is a clear security risk already gives them justification, but even looking past that, Apple is not the only platform that bars users from running third-party software or marketplaces on their products. For example, playstation, xbox, and switch all disallow running unauthorized games on their platforms. What makes Apple different?