It's like asking why the iPhone couldn't have both a touchscreen and a physical keyboard.
The whole point is for the touchbar to replace the function keys. It's essentially a souped up function key. Having both defeats the purpose of the touchbar in the first place.
Is that the point of the touchbar? Having used it for 2 weeks its not a replacement of the function keys but an extension of programs.
ESC , brightness and volume are the only keys that remain constant... Might as well keep those keys Physical. Its kinda annoying how they move .....