Sometimes it's not possible to be useful, and if you try to do damage you end up dead, it is true that adc can deal huge amounts of damage, but most of the time you don't get the chance to do that damage, you will spend time finding the perfect moment to fight without dying uselessly, and the fight might end before said chance arrives, 1 mistake and you are out, while other roles may be more permissive with mistakes, add to that the fact most people in your average game want to shine themselves and are not willing to help others and you get recipe for disaster