Why are Bite and Crunch Dark-type moves? Biting seems like a pretty basic attack, so why aren’t they Normal type?

To put it in short, for the same reason Twineedle is a bug-type move, despite the fact that it’s just the act of stabbing something with a stinger: it’s not just about how basic a move is but also with which element is incorporated into its use. Bite may look like an ordinary bite, but it’s only truly the Bite technique if it also involves imbuing the user’s teeth with dark-type energy. Crunch is the same way but in larger, easier to see doses. (This, plus the fact that the dark type was discovered after Bite’s registration as an officially recognized technique, is why it was originally classified as a normal-type move, by the way.)