"Because the plot needed it so".
That's the default answer for some questions on the Headscratchers 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 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).
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