Because a good villain requires you to at least, in some respect, sympathize with them... Doc Oc had an actual character arch, and you see his struggle and plight. Even though he is in the wrong, you sympathize for his loss, and ultimately come to understand his reasons as he reconciles with his choices in the end... Someone once told me that the best villains are misunderstood heroes. They are heroes in their own right, fighting for something they believe is just. Their methods and morals may differ, but their struggle is often of equal importance to that of the hero. While I'm not sure that I buy that argument in the case of every antagonist, there is no doubt that such a perspective can make a great villain.
My guess is that villains in general are probably less understood (and less important) to movie writers, and executives than their heroes are. It's much easier to focus on, and advertise, central characters, while villainous characters (even 2-dimensional ones) can provide enough conflict and threat to keep the plot moving along. As much as I enjoy Marvel movies, most of them feel like focus-tested products to me more than artistic projects... million dollar movies demand million dollar profits, and when you've got 2 hours to flesh out an origin story, and create enough set-pieces and explosions for a sweet trailer the lowest common denominator will often do. It just means that great villains are that much more special when they come around.