It is, theoretically, possible to distribute without the App Store, but users need to activate Developer mode and do quite a few manual steps. It's not a click (or drag and drop) like FF and Chromium Based browsers.
Apple is the only place I have to pay to develop. Apple is a luxury product. Apple users can pay luxury prices for my time.
At the same time Chrome Web Store is not suitable for paid apps, today it looks like a big dump: it is filled with outdated and poor made extensions. Raking system makes good apps hardly discoverable. Also paid Chrome extensions work only in 36 countries. Dark Reader owes its popularity to Hacker Vision extension that became paid some day. Maybe that's the reason why some developers prefer monetizing their extensions by selling users browsing history.