AZ:
Because they are afraid to fully-flare the airplane. The T-38 buffets like crazy in the flare (and even on final) as it is a thin-wing airplane, so to max perform it, you have to fly in buffet (actually, downwash hitting the horizontal stabilizer). Wing root is about 5 inches thick; wingtips are about 0.95 inch.
It takes a lot more pull on the stick than the student expects, it buffets a lot (a buffet that in any other airplane you would instinctively think "Cripes!! I'm in a deep stall!!"), and the best part is, if you fully flare it, you can't see where you are going. Kind of like landing a tail-dragger--the nose blocks the view. So, they incomplete-flare it, and we touchdown fast. It is hard on the tires--they rarely last more than 25 landings, and that is going down to the 4th layer of exposed tire-cord. (It's colored red so the crew chiefs/pilots know when it is time to be changed).
Viz from the backseat is bad (but not as bad as the backseat in the F-4). I know when the student has a good aimpoint on final when I can't see the aimpoint (His helmet is in the way). You learn to use a lot of peripheral vision, but today's kids were raised on X-Box and Play Stations--to them, the world is always straight-ahead. And they won't look away from the HUD--the electronic God of wisdom.
Related: if I say: 'Traffic at 3:00 O'clock, 4 miles, 5 low," odds are he'll look the wrong way. It became common enough that I wondered why. Answer: starting about 5 years ago, all my students grew up with digital watches. They had never owned an analog watch, so clock-position means nothing to them--they have to learn it when they are 23 or 24.