But why the instructors? Sadly, in many cases, because they were fed that by their instructors, and they haven't thought about it since. And because they've developed a method of teaching, and just hate to deviate from it.
Learning to fly accurately before trying the circuit is IMHO even more important in the case of helicopters, where you have more to do and usually a smaller circuit. I certainly used to try to make sure students were reasonably good at upper air work before getting them in the circuit. I also tried doing this on short cross countries; it relieved boredom, and reminded them why they were doing it all..."There's a hill, what do you think you should do? Climb? Good answer. Go ahead then..." Yet I remember arguing just this point with an instructor who was far more experienced than I was at the time (and who was supervising me at the time), who more or less seemed to think that students could learn to fly in the circuit!!! I said I couldn't teach that way and we agreed to differ, but he was convinced that they learned faster his way. But did they learn better? Well, I talked to some pilots who'd been taught to fly by this guy, and more than one felt they'd been rushed too much when learning the basics.