Originally Posted by The Question
But making Stark or SHIELD Ultron's creator would bring it back.
Except you do it at the cost of Hank Pym's character. You effectively strip away his depth, and guilt when he's responsible for Ultron (certain continuities showcase him being responsible for building it, others take it one step further by showing how Ultron is the personification of Hank Pym's darkness). That is honestly more interesting to watch than another "experiment gone rogue."
Though I must say, after rewatching Avengers, it seems like Ultron was heavily foreshadowed as a method for dealing with huge scale events like the Chitauris coming to Earth (repeated dialogue featuring Phase 2, and how Phase 2 was designed to stop events like Chitauris).