Unfortunately my guess is two reasons: first, the student is chomping at the bit to et soloed so they dont need the instructor and buddy cord anymore. (Me was dis way)

Second, many many people learn as I did, just from someone with the skill (hopefully) who happens to be at the field and willing to buddy box you a few times. They want to fly there own planes as well so they might be inclined to skip a few areas to get you through the basic take off, fly your plane and land it steps out of the way and cut you loose.

I doubt this was why my instructors did not teach me, it was more likely because I had multiple instructors, and they couldnt all know what I already learned. I did ok on my first only because I had seen a couple and knew not to try to get back to the runway if I couldnt make it.