The difference is that the extension platform itself is open. I can compile Chromium and create/install/redistribute extensions all without ever asking for Google's permission, and still have an acceptable user experience on non-Windows platforms. Afaik, Edge doesn't allow easy extension installation prompts from third party websites.
Yes, redistributing extensions for Windows Chrome users (not Windows Chromium) does require the Chrome Web Store. This is unfortunate, but at least Google has a better track record when it comes to banning extensions that modify their things on their own web properties.
Compiling UWP apps such as Chrome extensions doesn't "require Microsoft's permission" either and you can sideload them as well.
However, as you point out, Google prohibits side loading for the majority of real Chrome installs.
The time since I last found a malicious extension in the Chrome Web Store is five days[0]. They're terrible at policing this, which is a point I've actively complained about for years.
Yes, redistributing extensions for Windows Chrome users (not Windows Chromium) does require the Chrome Web Store. This is unfortunate, but at least Google has a better track record when it comes to banning extensions that modify their things on their own web properties.