I think there is a balance that you need to have. You want to know the villains motivations but they don't have to be sympathetic. I frankly don't mind if the villain is just evil and someone the hero can't reconcile with. Not every villain needs to have a fleshed out backstory or sympathetic background in order to work well. Audiences will accept a villain just being a bad guy. Further, audiences accept a villain full formed ala X Men films and the Nolan Batman films. Audiences aren't automatically expecting every villain to have an indepth back story or to have to understand why they're bad. Its really not that complicated, assuming the villain and the story are good.
Basically , alot of the sympathetic portrayals and full on backstories are hold overs from the Burton/Schumacer Batman films in which studio intentionally cast big name actors because they felt the villains were more important than the heroes. As a result they wrote in back stories and origins to give more screentime to the big name actors. Alot of the sympathetic villain arc comes from Spiderman 2 and Sony just kept trying to redo it over and over.
Personally I think the next Spiderman villain needs to be someone who is truly a villain and not another misunderstood, sympathetic, scientist slash father figure. Characters like Kingpin, Vulture,Carnage, Kraven, Mysterio, Chameleon , Venom, and Doc Ock usually fit that bill.