Where did the term "mono block" originate?

Not sure of the origin, but the term itself is my 2nd favorite part of having mono blocks. 😎

Why isn't or when is a stereo amplifier a "stereo block"?

The word comes from outside of audio, and probably comes from manufacturing. 

Originally all amplifiers were "mono blocks" but they weren't called that.  THey were just amplifiers.  At this time stereo was not real and 1 amplifier was all you needed. :)  See if you can find some vintage tube amps made for the early motion picture theaters. 

Then stereo came out (1930s per https://en.wikipedia.org/wiki/Stereophonic_sound#History)  and they were just "stereos" and stereo amplifiers became the normal way of packaging an amplifier. 

So the term monoblock applied to an amplifier probably came at some time after that.

                                          Semantic Gymnastics!

     Perhaps: because some think of various system topologies*, as being built of individual, "blocks" (syn: pieces, parts, components) and the long version is, "monoblock amplifier", as opposed to, "stereophonic amplifier."

  "Not sure of the origin, but the term itself is my 2nd favorite part of having mono blocks."

                                                     Me too!

       Suppose LEGOs had preceded blocks, as the World's favorite building unit: mono-LEGO amplifiers?    

                               Just doesn't have the same ring to it!

*: https://www.ooberpad.com/blogs/audio-video-tips/what-is-the-difference-between-a-regular-amplifier-and-a-monoblock#:~:text=A%20monoblock%20amplifier%20is%20a,share%20a%20common%20power%20supply.

I got a pair of Krell KMA 100s back in the early 80s and back then they were just mono amps. The block came after. I cannot say where it originated. They were more squarish than stereo amps and instead of putting them in cabinets we stuck them on the floor either behind or between the loudspeakers. They looked like blocks so the term is not a stretch. I now have mono amps that are under the speakers and are in no way shape or form blockish. They look more like a whore house chandelier:-)