It is not. The point here is - its easier to scan liquids when they are all together in a plastic bag vs. scattered/hidden under your stuff. You'd be surprised to know what these airport x-ray machines can do.
They can detect organic/inorganic liquids and advanced ones can also accurately guess the atomic number (based on x-ray penetration) and then infer the presence of explosive chemicals.

Its easier for the tech guy to do this if all liquids are in one place