All I read was that it can directly connect two PCs, even if they are behind a NAT, which sounded like a cool idea at first.

But having random sites doing this, didn't feel right to me.

The real issue is that WebRTC ignores your proxy settings. This is obviously incorrect behaviour.

Also there's no reason to allow silent P2P connections.

If you're thinking of the bug that exposed private IP addresses to the browser, it was fixed years ago.

It's not fixed, see your addresses at https://www.privacytools.io/webrtc.html

If you want to force all traffic through a proxy, you must enable it using an extension, such as this one:

https://chrome.google.com/webstore/detail/webrtc-network-lim...

There isn't a solid UDP proxying story for browsers, generally.

Is there a technical reason for this?

Yes: it's not possible. Neither Socks(Tor) nor HTTP proxies forward udp. Only VPNs - which is an OS-level configuration (normally anyway)