"Because the plot needed it so".
That's the default answer for some questions on the Headscratchers section. Questions 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 Smarty 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).
See also the Anthropic Principle and the Theory of Narrative Causality, and always bear in mind the MST3K Mantra.
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.