Default Answer

That's the default answer for some questions on It Just Bugs Me section. Question such as:

Why didn't Anderson do Y instead of X, that gave chance to Ben to foil his plans?
Why didn't Alice just tell Bob that "Y" meant "You", not "Year" right at the start?
Why didn't the villain just shoot the hero with a gun?

And if you want to go Smarrty Pants on us, the more cynical version of the answer is: the writers decided to put it there (and as far as fiction goes, they are gods).

Except if you're in Real Life. Then the answer may be:

Reality Is Unrealistic.
Nature just doesn't work that way.
"People are tricky"—Aimee Mann
Because you touch yourself at night. "When I think of you, I touch myself." - Divinyls
People are stupid.
"Donald, when someone asks 'Why do they?' or 'Why don't they?', the answer is almost always, 'Money'." - Maureen Johnson, in Robert A. Heinlein's To Sail Beyond the Sunset

See Also: Idiot Ball. Yeah, now you know. And Knowing Is Half the Battle.