That is a good question. It would shrink the size of the cable ends quite a bit. Without the cable ends, you can get a REALLY sleek-looking setup, with the cables running close to the tubes.
My guess is similar to JoeNation, the cable end allows for reinforcement and standardization of housing. There is quite a bit of variability of “4mm” shift housing. I have measured 4.1-4.3mm
Shimano recommends using these styles of ends (see below) whenever possible. I could see that being an argument for cable ends.
