the question is why is representation so good?
I am not so sure by the answer:
its a the best non war civic and better then HR.
I think you are supposed to abuse MIDS and go SE
This is a little simplified, but with specific reference to the Mids...
Players often run numerous specialists in the early game to (1) build an Academy; (2) win Lib; and (3) upgrade units via Great Merchants.
Building the Pyramids means that all of these early Specialists dedicated to generating Great People receive +3
Now, that doesn't sound like a lot. But +3

per specialist goes considerably further in the Classical/Medieval period than, say, the Renaissance. For example:
Calendar: 350

Rifling: 2400
The Mercantilism civic is also often seen as weak, because you lose foreign trade routes in exchange for just one free specialist per city. However, if you are running Representation at the time, the loss of commerce is offset by the free beakers. You can gain extra production (by running Engineers) and deal with the loss of foreign trade routes which occurs when the AI inevitably switch into Mercantilism
en masse.
After reaching Military Tradition, Rifling, or Steel, I (at least) tend to stop running specialists in favor of whipping or drafting. This is the point in the game at which Representation becomes available to those civs that didn't build the Mids. It is therefore the time at which the science boost provided by the civic is least useful. (Again, a big simplification.)
I should add that I hardly ever build the Mids. I only consider building it with access to Stone, and even then I usually prioritize claiming good land. It is my favorite wonder to capture, though. That being said, I don't play SGOTM, and the players there are much more informed about these things than I am.