IF there weren't App Stores or anything, it would be possible to download applications from websites.
So basically you would distribute the same way as in PC. You buy the install files in physical/digital form and install it. Then enter cd-key or something.
And of course somebody would come and make some kind of store to make the purchasing easier just like on PC. (for example Steam)
And yeah. Its ridiculous when Apple/Google take shares of the profits, comparing to what UE4 for example takes.
Apple gives you a market place to provide marketing, handles the "do I trust them" problem, makes the pain to pay nearly non-existent (another CC form?), and handles the installation process for you. Ditto for most other platforms.
The platforms are more acting like Publishers than frameworks now, so directly comparing these prices is a bit hard.
They have hundreds of millions of credit cards on file and handle the billing and payments. If you want to run your own login and payment you are free to do so, this is exactly what we do now, although on smaller apps we gave up the 30%.
Unreal / Cryengine = 5% (Negative responses of high cost) But they actually provide the tools and are makers
Apple/Google = 30% for providing an app that lets you support their OS (What's an OS without Apps)