Why can't the touch layer on the screen can't be replaced? AFAIK, touchscreen has three different layers: the protective glass, the touch panel, and the LCD screen:


Touch technology in smartphones explained - FlatpanelsHD

However, when an area of the screen is not responsive, in my observation technicians will replace the whole screen, not just the touch panel, and this would increase the fixing cost significantly. Why is that?
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.