"Plot Induced Stupidity".

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.