Because users understand what “microphone access” entails. “Use WebRTC?” means nothing to the average user.

Fair point, but "cookies" didn't mean anything to the average user either, and "cookie consent" banners are the norm now.

I think very few people would argue that cookie consent banners in the form in which they are the norm are a good thing just like permission prompts for microphone access are.

Mobile apps require location permissions to use Bluetooth right now, even though that's a hard to understand situation for average people.

If a feature can be used to track people, you have to flag it off or else you are just contributing to the tech Big Brother apparatus.