Steam and EGS don't just offer payment processing. Bandwidth isn't free, neither is patching, distributaion and "curation" (albeit this one is definitely more debatable).

Even then, payment processing isn't just about taking a $10 transaction. A single dispute on Stripe will cost you $25 whether you are succesful or not. Handling payments in non-local currencies is 3.5% + 20p on stripe, plus currency conversion fees.

I don't think 30% is the right cut, but 2% isn't a fair comparison for the service.