Fuck the stupid morons who defend Apple.
Imagine if Microsoft banned Windows users from installing the software they want on their computer.
Imagine if Microsoft required all software developers to give them 30% of their earning or Microsoft will ban them from Windows
The difference is availability of choice. On apple phones, Xbox, Nintendo, and PlayStation you are locked into a single source of software. On a PC there are myriad of game stores you can choose from. Sometimes you can even buy the software directly from the developer. Usually people are upset when this choice is taken away (for example epic exclusive games). Nobody would bat an eye if a developer offered their game on epic or their own platform with a ~20% discount compared to steam. But it is up to the developers to make their game available on any of the PC game stores.
In conclusion, steam is not a platform holder, they could charge whatever they wanted. If the markup was too high, you could simply choose to buy your games elsewhere. For most people, this 30% is worth it for the features and buyer protection that steam offers compared to other platforms.