Valve appear to even be complaining about the 30% fee, that is _equivalent_ to their own fee.
The issue is that the play store is shipped as the default and many people do not know there are other options.
This can be difficult since in the short term it's easiest for every market participant to just use the Play store. Creating a new market takes upfront $ and is a risky business.
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.
You absolutely can release Android apps without going through any app store, and if you go through an app store, it doesn't have to be Google's. There are different pros and cons for Google's store, handset maker’s stores (e.g., Samsung has their own), other stores (e.g., Amazon's), and direct distribution of your own apps without a store.
Android technically allows for the same, see the existence of f-droid for example, but the experience is less smooth (more dialogs to tap through) than when using Google Play.