An outdated grammar rule: the subjunctive
Well, it's simply because we're in the subjunctive mood.
"So what on earth," I hear you ask, "is the subjunctive mood? It sounds like a form of deep depression!"
Frankly, that's a good way to describe the effect it has on me. It pulls me right out of the story. Anything that pulls a reader out of a story has to be a bad thing. If it also puzzles a child reader, it's a positive crime.
The subjunctive mood is quite simply a form of the verb when a writer stops dealing with real things about which we can argue ("The King is alive", "Mary is here") and starts dealing with uncertainties such as wishes, commands and unreal circumstances ("Long live the King", "if only Mary were here"). We're so accustomed to "If I were you" that it doesn't sound so odd-- though it baffled me as a child; nobody ever explained why I was suddenly (and impossibly) more than one person.
The subjunctive is an outdated grammar rule that should have died a natural death a long time ago. I'm convinced it's only the extreme pedantry of editors of children's books that has kept it alive. It has no plausible reason for its existence in the 21st century. I found the following (from Margaret Mahy's Underrunners--a book riddled with subjunctives) positively pompous.
"As if she were being scribbled out."
"As if it were making a grand announcement."
There's another good reason for killing off the subjunctive mood: too many writers use it incorrectly:
"Mary asked if it were his idea."
"If she were French, her accent didn't betray her."
Examples of incorrect use from published books--incorrect because the sentences are not in the subjunctive mood:
"But if it were in the hydro it was well hidden." (Since "it" is referred to here in the plural as well as the singular, this sentence is extremely clumsy.) From Plague Ship by Andre Norton
"If he were wrong, we might then have an excuse to call this crazy thing off." From A Killing Frost by John Marsden
Any editor who dares suggests I turn a pronoun (or proper noun) into plural just because I am using the subjunctive mood is in for a hard fight with me.