Because it's compromises. Having a touch display would just mess up battery life, besides that what practical use could it have? If you use it like a normal laptop would you use your finger to touch the screen? You can't do so for a period of time, you'll strain your arms, not ergnomic at all. If you wanna use a touch screen or a tablet for drawing with the pencil for example, it's best to have it on a flat surface like a piece of paper or somewhere holstered up on your lap/knees while sitting like a sketchpad. You can do that perfectly with an iPad Pro and with iCloud and a plethora of sync-options your workflow can still be close to seamless even if you work on two devices. If you force the MBP to do all of that, it's not good in any use case. Battery drains faster for a screen probably a very small percentage of people will use at all, even if you can fold back the display, it'll be an uncomfortable thick tablet...
All those "2 in 1" computers, foldback stuff etc... it's all just not very professional and by that I mean: how MUCH do you use those features? A touchscreen on a hinge in front of you?
It's just crap really.
Touch input is best for very mobile or thin devices being laid down or held close to your body, nothing you have to reach out to. Also I really see no use for touch you can't do better or more precise with a stylus or a mouse.
Erm....well, battery life may be reduced, then again, it may not.
Practical use would, obviously be for drawing in drawing apps as well as navigation in app's such as Z-brush, Maya etc...thought that was self evident!?!?!?
Do you use your finger on your phone/tablet?
You might use your finger to touch the screen, yes, of course.
I use my finger to touch shortcuts on the keyboard and the trackpad...what's the problem in having another input?
If the screen folds right back, like the SurfaceBook, then you can put it on a flat surface, or your 'lap'...some may find it uncomfortable some may not.
The OL' 'not professional' argument doesn't stand up does it?
If I were to use this 'hyperthetical touch Mac' to draw a comic or illustrations, make 3D models and get paid for that then it's professional...right?
Ok.
You don't much fancy the idea and have given some pretty weak reasoning as to why not( namely the battery might be less and you, personally, wouldn't have use for it.
...other people would find such a device highly productive and would have use for it.