I think it's partially because vertical touchscreens are ergonomically terrible. And, with the emphasis on thin and light, the addition of a digitizer would create thickness they don't want, and the use of said touchscreen would cantilever the base of your notebook off the desk and slam it back down repeatedly.
Plus, the usage of a touchscreen on a keyboard-centric device just sucks. I'm typing - oh, now I have to reach full arm at the display with my wrist at a weird angle, now I have to find the home row again, now I have to reach full arm extension again, etc. With a trackpad, you can just move one hand an inch or two, and be right back where you were. And good luck trying to actually hit some of these checkboxes / window widgets with your meaty appendage - most of them are less than 32 x 32 px.
There's a reason why a lot of people like the "cat tongue" / "mouse nipple" on Windows laptops - you can use it with even less hand movement from the keyboard, and far more accurately. I'm hoping that touchscreen laptops are nothing more than yet another fad that will go away.