If steam can charge 30%, why can’t Apple/Google?
You can release an executable that can run on Windows, Linux, or OS X without Steam. You can't release an executable that can run on iOS or Android without going through the App Stores.
Also the friction of buying is lower. I’m not going to put my credit card information in every random website. I will keep a credit card on file with Apple and just use my fingerprint to authorize the purchase.