WebRTC does not use real SCTP but a rather a tunnel over UDP. Besides, it's not like WebRTC accounts for a significant amount of Internet traffic. I'd say SCTP sees more usage simply because it has been around for much longer with SS7 being the primary use case.
There is a standards track RFC that defines SCTP over UDP. RFC 6951 "UDP Encapsulation of Stream Control Transmission Protocol (SCTP) Packets for End-Host to End-Host Communication".
Sure. But it's still not real SCTP if the IP header's Protocol field doesn't say 0x84 :D