Part of WebRTC's requirements [1] is to be fair on competing TCP connections.
This is a very good point, because I agree with you that in general parallel connections cause each connection to have less throughput, but if those connections have a mutual goal to download the same content it can actually increase the overall throughput than downloading that content via a single TCP connection. That's especially true when those connections are to different sources which can workaround specific network issues you might have in a single connection.
Actually the value proposition for our customers is to improve the QoS. And we've shown that's the case in customer case studies.
[1] https://wiki.tools.ietf.org/html/draft-ietf-rtcweb-data-chan...