Why can't the touch layer on the screen can't be replaced?
It depends on the phone, really. I had a Motorolla Attrix HD with a broken touchscreen, the LCD was fine, and I was able to order a replacement touchscreen by itself. However, during the repair, separating the LCD screen form the touchscreen damaged the LCD (it was glued on way more tightly than the one in the youtube video I was following for this exact model).

Some phones, the touchscreen and LCD screen are sold as an assembly, other phones, the 2 parts are available separately.