I never really understood this complaint. UDP adds such a tiny and useful header on top of IP that it hardly seems meaningful to reinvent the wheel instead of just using it.
The only thing I get from that document that answers this question to some extent is the extra overhead of the checksum field in UDP headers. In other places, they generally don't seem to actually discuss the option of building a QUIC-like transport, that is a transport-layer protocol that still exists on top of the UDP skeleton.