What a reasonable question to be asked today.

Let me rephrase GP into (I hope) a more useful analogy. — actually, here’s the whole analogous exchange:

“A rectangle is an equal-sided rectangle (i.e. “square”) though. That’s what the R stands for.”

“No? Why would you think a rectangle is a square?”

Just as not all rectangles are squares (squares are a specific subset of rectangles), not all datagram protocols are UDP (UDP is just one particular datagram protocol).

What networks are you using without ICMP?

Presumably you're also using systems that don't support Unix Domain Sockets which can be configured as SOCK_STREAM, SOCK_DGRAM, and even gasp SOCK_SEQPACKET (equivalent to SOCK_DGRAM in this case admittedly).