Because it's a crap idea and design, I'll use a tablet if I want to poke at things. A touchscreen on a laptop is like driving a car with joysticks, it's not ergonomic.
Also, Apple never does things half-assed, if they put a touchscreen in they're doing for all models just like they did the touchbar.
Touchscreens are the big missing feature on macOS.
It's not useful as a primary input method, but after using a Windows Surface computer for some time I'm surprised how often the touch display is useful.
Lots of websites are optimized for touch / mobile first. Wether you are filling a form or watching Netflix or Disney plus, touching the screen is just much more convenient. Keyboard navigation is increasingly an afterthought on many websites.
Macs now support running iOS apps. Using them without a touch screen is going to be a very poor experience.
And finally, some things like annotating PDFs are things that are really cumbersome without a touch display -- when I need to do that on a Mac, I just print out the page because using the track pad or mouse for annotations is just not an option for me.
The laptop I bought for my dad have touchscreen "by accident", as neither of us recognized it as a feature to go for. Now he says its the best feature of the laptop, from pinching on maps to selecting checkboxes/radiobuttons. I dont know who was more surprised to find it useful.
It would also require the whole thing be designed differently.
The hinge would need to be sturdy enough to not wobble when the user is dragging or tapping with a finger. The body would need to be heavier than the screen by a big enough margin to not topple the whole thing over when tapping.
Edit: Ouch, so many downvotes just for asking for a feature. Weird.