(BTW: These topics have previously been covered in GQ.)
WWII was a huge mobilization so there were a lot of divisions created. With some exceptions, they were just counting when naming divisions. (The 82nd was the 82nd Infantry Division (Airborne).
These days, if the army wants to create a new division, it will look for the most decorated inactive division and re-activate it. (This was probably the last was deactivated.) These old divisions have an existing history around which the new division leaders can create an esprit-de-corps.