Why did Robotnik have to worry about ARK, when he could just build another Death Egg? They're both virtually the same thing.
Robotnik/Eggman only found out about the ARK when he went looking for his grandfather's secret weapon, which of course turned out to be Shadow. In turn, Shadow told Eggman about the ARK, informing him that it contained a weapon powerful enough to let him rule the world.

~ Crazy C Lea