Doesn't have to wrap it in UDP. Seems to be just an expectation that firewalls can't handle SCTP directly so you will need to wrap it in something else. Extending a firewall to understand SCTP isn't particularly difficult.
In any case, the SCTP reference implementation in FreeBSD can do RFC 6951 encapsulation, someone could add it to Linux too.
It's specified as wrapping in DTLS/UDP, so implementations have to do that if they want to be interopable. The "just an expectation" is sadly a well-studied fact.
That FreeBSD can do that is neat, but just it doing that is pretty much the answer why a library wouldn't implement it, or at least not as a priority.
You then need to try and connect via SCTP and then fallback to SCTP-over-UDP (given Windows/macOS don't support SCTP and you need SYSTEM/root access to create a raw socket to implement it in userland, which is a non-starter), which has a performance cost in a common case (falling back).
