We didn't see Rocky take on monsters like Fulton and Willard repeatedly, or contenders of size like Carl Morris and Firpo. With his lack of height and reach, opponents of this stature may have posed serious logistical issues for Marciano.
Marciano fought 6'2 213lb Joe Louis, who even at age 37, had far better fundamental skills and a much better left jab than any of those big men. 1951 Joe Louis beats all 4 of those men in shut out unanimous decisions.
Fulton was ok, but he couldn't take a punch, and was nowhere near as skilled as joe louis. Him beating a midget 5'6 Sam Langford who was way over the hill doesn't mean he could handle a prime Rocky Marciano.
Willard a monster? he was 37 years old, hadn't fought in 4 years. He looks terrible on film. A clumsy big oaf.
Carl Morris? Your kidding right? He was a big joke
Luis Firpo? One of the most unskilled fighters ever captured on film, most amatuers would beat him. He has a good punch, and that's it