Bike shops have housing for brakes and shifters separate. Why? They look the same.

They are actually quite different. The diameter of the cables are different. Additionally, gears are more sensitive to stretching and random exceptional situations. Therefore shifter housing is much higher quality, the housing runs in a straight line, so as not to bunch up the gear cable. Whereas brake housing is circular, it is very cheap to manufacture. I recall I purchased a 50 foot roll of brake housing for $5. Please make sure your bike uses shifter housing on shifter cables and brake housing on brake cables.