Why do manufacturers do this? What's even more interesting, many a bike have the middle cable fully housed, while the other two are "interrupted" as shown above.
IMHO those are the pros and cons of interrupting cable housing:
- more complicated to cut to length and install
- more friction as there are a lot of enter/exit points for the cable into the housing
- far more susceptible to dirt and water
- more expensive, because of end caps and labor cost.
As is evident, in my opinion, all cables should have uninterrupted housing. However, all kinds of bikes in all price ranges have these intermissions - on the top tube, bottom tube, seatstay (for the RD). And it is not a factor of weight, as 10-year old massive steel frames also utilize it.
There must be an explanation. But what is it?