Because three ounces of explosives aren’t enough to critically damage a plane. The TSA’s 3.4-ounce limit may seem like an arbitrary pain in the neck, but there is science behind it. After the Feds foiled a liquid explosive plot in 2006, officials tested explosives and determined the amount of liquid needed for a serious blast. In fact, 3.4 ounces is the maximum amount of liquid that poses little to no risk to travelers. So while chugging a soda at the security checkpoint is nobody’s idea of fun, at least the TSA has a good reason for confiscating rogue beverages.
Oh, and since you were going to ask: Yes, you can carry a pie through a TSA checkpoint, but the agency’s website warns, “[P]lease be advised that they are subject to additional screening.”