why do people vote ‘abstain’ on simple (to verify) edits?
They might use a view that displays all edits of a certain type that they have not voted yet (it’s what I do, I have bookmarked several non‐voted open edit searches).
So Abstaining is like they don’t want to see this edit in the list any more and they don’t want to interfere (for nor against) with it.
There are so many open edits that we usually vote on what matters to us in priority.
In addition to what @jesus2099 have said, I have sometimes abstained on an edit merely to follow a discussion on it, even if I didn’t have a clear position for or against the edit myself. (Of course, these edits are rarely of the “simple (to verify)” kind, so may not be applicable for this question. )