STUN doesn't relay. It assists with P2P connection setup. TURN relays, and is intended as the path of last resort. The vast majority of WebRTC traffic is P2P.
Many providers choose to use SFU's even when not needed because they have features like recording that require it. I've gotten much better quality and latency on normal webrtc than I have over slack, teams, hangouts, even over bad mobile networks.