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!