Here's why:
* It describes an action: Fidgeting is the act of making small, restless movements.
* It can be used in sentences with verbs: "The child was fidgeting in his seat."
* It can be modified by adverbs: "He was nervously fidgeting."
While "fidget" can also be a noun (referring to a restless person), "fidgeting" in the example you provided is specifically the action of fidgeting.