Its possible to change just the front glass on certain iPad's.
Unfortunately the iPad Air 2 & 3, iPad Mini 4/5 and the iPad Pro (all sizes) require a full unit that includes the inner LCD.
Basically apple changed the way that the screens are made, apple now "laminate" the LCD, which basically means that the glass is sealed to the LCD.
This has the effect that the iPad can be much slimmer, with reduced glare & reflections
If the glass breaks, it is not possible to remove it as a single component. The cover glass, touch sensor and the LCD are bonded together and need to be replaced as a set at a higher cost to the end user.