They have a monopoly on providing services within iOS.
When an ecosystem becomes large enough, societally impactful enough, and is controlled exclusively by one entity, that qualifies as a monopoly within that ecosystem.
You remove any capacity for competition to drive down margins. Apple can ban Spotify tomorrow to promote apple music on their devices, for example. Not anti-competitive in your mind?
You have two gated monopolistic ecosystems, choose the one you want to be gouged by. In a competitive environment, margins get compressed close to cost to deliver. If a company can achieve huge margins on items that could otherwise be much cheaper to deliver in a competitive environment, that qualifies as anti competitive.
How much does it cost the supplier to host apps via CDN? Or to process a single payment? Apple can take as much margin as they want on these because nobody else is allowed to provide those services within their ecosystem
The law will adapt to this, it's quite obvious to those that are unbiased.