In Amazing Spider-Man, Doctor Conners gets a lizard-like tail, body and face after injecting himself with a serum made from lizard DNA. Why didn't Spider-Man get eight limbs and a face like a spider? Is this is because the villain injects more lizard DNA or he is injecting something else as well?
In the movie version of The Amazing Spider-Man, Peter Parker is bitten by a genetically modified spider. It is hinted at, Parker's father may have experimented on Parker as a child. The bite may have simply activated the genetic patents started by his father.
Since it was a bite, it did not transfer, nearly as much DNA as Doctor Conner was happily injecting himself with for a number of days but was never able to keep the transformation stable. Parker's transformation however, appears to be permanent, leading me to believe there is more to his transformation than initially believed.
In the comic medium, Spider-Man has had several brushes with transformation into more spider-like forms over the decades. None of them have agreed with him (or the readers).
The most obvious reason Peter Parker is human is because we identify better with him if he is. It is much harder to identify with a character if he is much less human in appearance. Spider-Man was transformed temporarily once due to a mutagen from the villain, Plantman. (see below)
Amazing Spider-Man Vol. 1, Issue 437
Parker has suffered other mutations as well, including one that ends up giving him six arms! This was a side effect of him trying to remove his powers, instead he expanded on the problem.
Six Armed Spider-Man - Amazing Spider-Man 100
As you can see, neither look works well for him. He would be much less likely to elicit sympathy as a character, and a secret identity is straight out the window. It is rare for a heroic character to be given such a horrifying appearance because of the challenges it makes to address his normal life.
You will have to content yourself with the idea that Spider-Man's human appearance is best for us as readers and for comics as writers. They may experiment, but they are not trying to make too great a change in the basic character. But maybe one day, they may try to work with this idea:
This mutated Spider-Man is the complete transformation you were asking for. But I suspect there might be others out there who are seeking the complete package just like you are. Enjoy!
I think the real reason for this is just storywriting, but at the same time a bite of a (common) spider won't inject any DNA, just some poison (and in the case of Peter Parker: radioactive mutation magic).
Keeping Spider-Man in human form has several advantages:
- It makes it possible to keep his non-superhero/commoner appearance intact, otherwise removing many, many plot lines (e.g. everything involving family & friends).
- He's less like a "monster" and people have an easier time to identify themself with him.
- Similar to the previous point, giving him real spider features would make the whole setting less attractive for people with arachnophobia (quite a lot, seriously).
- This makes it easier for people to decide on who's the actual bad guy: If you look at other villains, most got stuff making them significantly different from normal people (mutated body, mechanical arms, etc.). Venom is also an interesting example for this: At first, he appears just like regular Spider-Man (or any guy in a full body suit), later on, when the evil nature is revealed, his face/mouth changes to appear a lot more alien and less human like.
Also, I think I actually remember one episode of one of the older animated series (either 80's or 90's; later air date in Germany, so might be off here), where he mutated further for some reason I don't remember, i.e. he got more spider-like features, but in the end he's been able to overcome these.