Why is editor information presented so prominently next to the original poster's?
The edits are sometimes as trivial as a capitalization with some spaces inserted, and yet the editor is presented as though he co-authored the question.
Simply because everybody can edit questions (with an approval process for anonymous users and people with low reputation).
When everybody can edit, that also means that the community moderation has to be able to review the edits. It's important for people to check whether an edit actually improved the question, of if it defaced, invalidated, or otherwise strongly changed it.
Making the fact "someone else edited this" obvious helps this community review proccess.
It also has the advantage of very prominently showing to new users that wiki-like editing by "random strangers" is a major part of how the Stack Exchange sites work; this is something that's often unexpected to people coming from "normal" forums.