Does using powdered sugar instead of granulated sugar make cookie dough drier?
This answer touches on the problem:


  Superfine sugar will dissolve too quickly and won't allow enough air to be incorporated.


Powdered or superfine sugar will still give you the same sweetness property as the granulated sugar. However, the step of creaming butter and (granulated) sugar is not just for mixing. It also incorporates some air into the fat; a well-creamed mixture will look "fluffy" and paler in color.


  In creaming the butter and sugar together, you are using the sugar to
  aerate the butter and fill it with bubbles that can capture the gasses
  released by your leavener. The more fine bubbles you have in your
  network, the lighter in texture your cakes will be and the finer the
  crumb. This is true for your muffins as well, while it makes your
  cookies light and crisp instead of hard and dense. (King Arthur Flour blog)