*blinks in Grima stabbing Saruman*
Saruman was a maia shoved into basically a human form, much weaker then he was in Valinor. Same with all the wizards, they gave up almost all of their natural abilities when they were put into their wizard forms and went to Middle Earth. They are very impressive compared to men, dwarves and most elves, but they're only a shadow of what they originally were.
Sauron, especially before he makes The One Ring, has his full might from before Middle Earth even existed. He was the servant of Morgoth because Morgoth was a step above him on the (for lack of a better term) cosmic scale, as are all the Valar, but on Middle Earth, with Morgoth long gone, no single entity would be as powerful as sauron, especially before he split a lot of it off into his ring.
The closest Sauron comes to a physical defeat in the lore, pre rings, is when he fights Huan, a hound of Valinor, in werewolf form. But Huan was a very special hound, unaging and sentient, and basically prophesized to only die in one specific way that Sauron didn't match. Even then, the hound needed Luthien (the daughter of an Elf and a Maia) to help distract Sauron, and they basically reached a point where Sauron only surendered so he wouldn't have to waste time making a new physical form.
I just don't think that one guy stabbing him with metal in the back reaches the level where it could kill his physical body, especially when in the lore the only threates to his physical being were basically: a hound of the gods double teaming him with the help of the daughter of an entity that would be his equal in power, a massive army vs him alone (and even then he probably only surenderred to get access to Numenor), and a cataclysmic disaster brought about by Iluvatar (aka God).
Now after he makes the One Ring he puts so much of his power into it that it fundamentally changes him, its probably the reason that he can't retake a fair form after his physical body dies with Numenor and just becomes some armored up monster, but before that his body was not something that could just be killed like an average persons. Of course in the lore no one should be able to trick him and stab him in the back in the first place....