Because you are choosing the random selection outside of the while loop.
Move that part of your code to the beginning of the while loop and it
should choose a new selection every time the game starts over.
word = random.choice(WORDS)
position = random.randrange(len(word))
jumble += word[position]
word = word[:position] + word[(position + 1):]
You are going to need to change the condition of the while loop since
word won't exist yet when it checks for it. Something like
playing and have
playing be a bool that you can set to
false so you can end the loop for whatever reason.