I agree with you somehow. 30% sounds too high. 10% is much more reasonable. Imagine you selling a house and the realtor getting 30% of it. Crazy high.
Source: https://www.theverge.com/2018/11/30/18120577/valve-steam-gam...
https://www.zdnet.com/article/microsofts-windows-10-sweetene...
No one is angry about cuts from app prices (or prices that are effectively upgrades of an app e.g. from trial to full).
What I'm angry about is when apple wants X% of the price of content whhen they don't produce the content, they don't process the transaction. All they do is host the store where the $0 client app sits. I don't think it makes sense.
And Epic takes 15%, (some of which goes to Epic's 40% owner Tencent).