Do they always wear the same mask?
From the website : http://tokyoghoul.wikia.com/wiki/Mask


  Masks (マスク, masuku) are worn by ghouls to prevent their identities as
  "humans" from being discovered by the CCG. Several masks bear a
  symbolic attachment or representation to their owner. They heavily
  influence the alias of some ghouls filed under by the CCG.


Ghouls wear those masks because they want to live alongside with the humans without the fear of getting caught by the CCG. This is the only reason for wearing the masks. They aren't afraid of being identified as specific Ghouls while fighting the CCG. Changing the mask will be like changing your very identity. It will be clever for a Ghoul to change masks from time to time but as mentioned earlier, it's difficult to change your identity (and everyone is proud of being themselves).
This seems to be the only explanation!