While it is technically possible for them to replace only the glass it would be more time consuming than it is worth. Since the glass and lcd are bonded on most phones, replacing only the glass would require then to separate the panels and reglue a new piece of glass. This can end up breaking the LCD and when it does work the results usually are not as good as new. In the end it is more reliable for a big company just to replace the whole unit, however if you were to replace the glass yourself you could save some money. From someone who has replaced the glass only on a galaxy note 2 though, I would say it isn't worth it if you are going to be using the phone as your daily driver.