grammaticality - Why is "extremely longer" not correct (according to the ACT)?

The sentence that included the question was:

• 
  

  In addition, LEDs last far longer than standard bulbs.

  
  

Question: Which of the following alternatives to the bolded portion would NOT be acceptable?

F. considerably

G. a great deal

H. extremely

J. much

Answer: H

"Extremely" is an adverb (and not materially different in definition from the other answers available) and should be able to modify the adjective "longer", correct? I don't have an answer explanation, so it's unclear why they chose this. Can someone please explain why this is the answer? Thanks!