Because Doctor Octopus's arms, which are mechanically constructed, are resistant to being hit.

Dr. Octopus is an American Marvel Comics super-villain who debuted in The Amazing Spider-Man #3 (July 1963), co-created by Stan Lee and Steve Ditko.

Characterization

Dr. Octopus' name was spelled "Octavious" when he first appeared. Despite his octopus-like name, he would rather be human. Dr. Octopus is a short, fat man with four powerful mechanical tentacles. He is a brilliant mad scientist who wants to prove that he still wants to kill Spider-Man. Many consider him to be Spider-Man's most formidable foe. Like Green Goblin, he is a former friend of Spider-Man.

Dr. Octopus' body was severely damaged during a long battle with Spider-Man in his later years, so he swapped bodies with Spider-Man in Spider-Man: At World's End to become the Ultimate Spider-Man.

Vowed to do better than Peter, building an army, threatening the Mayor (for commissioning him to kill the Spider-Slayer), and imposing surveillance on the entire city, at the cost of falling out with the Avengers and breaking up with Mary Jane; falling in love with Anna Maria (for whom he blocked a building), helping Peter get his doctorate, setting up Parker Enterprises, and helping Aunt May with her artificial skeleton.