Why? To make it harder and kill you, that's why. Literally the only reason I can conceive of given how traps are overused. Why put things somewhere for safekeeping and then make it so you are almost certain to die retrieving it yourself. It's not like there's an off switch or something.
Well I hope that the beings that put the traps do remember where they are thus they know where to place a cover. The other people have to pass a perception check to identify the origin of the gas.