As an instructor at my flight school it was indeed a bit difficult to get XC time during a regular lesson. Typical day I had 4 students. Each student was blocked at 1.5 hours of flight time with .5 allowed for prebrief and debrief. Fitting a XC in there would only delay the aircraft return for the next student as well as the start of my next two hour block. I tried to make it work, but I would usually end up sacrificing the quality of the lesson to get the time in my logboook. Instructors did it anyway, but usually involved a bit of creativity and flexibility on the students part as well a flexibility with aircraft availability.