The thing is, I very rarely (or arguably never) have a use case which requires that one client has multiple connections to the same server. The thing I want is almost always to have a bidirectional stream of messages where messages arrive in order. Essentially a simple message envelope protocol on top of TCP.
For sure, if you want an ordered/reliable stream then WebSocket is ideal. WebTransport is useful when you also want prioritization and semi-reliable networking, similar in concept to WebRTC data channels.