Because they are so lazy, so, they better say that something is dead, instead of taking action to learn, test, and rank.
Discipline, focus, and acting is something that is missing nowadays from the youth and to make their self feel comfortable, they come with a conclusion like SEO or any other internet marketing form is DEAD.
As long as Google will exist, SEO will be there, alive and pretty strong, the only thing that is worth to mention that we should adopt with any change that they adapt to their search algorithm. Adapting is part of life "Be like water", what a great quote to live by.