There are a huge number of different companies and service contractors bringing different things into an airport terminal.
Airport security cannot check every pallet of bottled beverages or every truck bringing in food service items for the restaurants that exist in an airport terminal.
They do screening of the workers, but controlling every source off fluids coming into an airport terminal well enough to be sure someone could not smuggle something in isn't really possible.
Gee, you're right. I bet there's always going to be some way to work around any restrictions they come up with. We should just give up hope and remove all the restrictions. Maybe we should even hand everyone a small bomb when they get on the plane so everyone is fully aware that any illusions they have of safety are hollow. /sarcasm
These are temporary restrictions in response to a specific threat which they have reason to believe is imminent.
These restrictions are not sufficient to prevent every possible way that someone could smuggle a liquid explosive on a plane.
However, they do have a reasonable chance of forcing terrorists who were planning something in the next couple weeks, to delay and adjust their plans. That gives the government more opportunities to catch them.
Throwing off their plans at the last moment increases the chance they will make a mistake that alerts authorities.
These restriction don't make a lot of sense long term. Restrictions on carrying on unsealed bottles of liquids existed not long after 911, but had been removed.
However, as a short term response to make what may have been a relatively simple task a significantly harder one, the restrictions are pretty reasonable ones.