I was surprised that the Shure SE535 was more clear sounding than a $700 Sennheiser HD-700 when I was shopping for a new pair of headphones. The big difference between the two is that the Shure SE535 uses 3 different drivers while the HD-700 uses only 1 driver. It might be the reason why cheaper IEM sound more clear / transparent than more expensive headphones?
Why don't headphones use multiple drivers? It's pretty common in speakers also to have multiple drivers.